Roll Node.js floor back to >= 24 (Active LTS) — closes #7779 (#7781)

* Roll Node.js floor back to >= 24 (Active LTS)

Closes #7779.

#7779 originally proposed bumping past the Node 25 stop-gap to Node 26.
After re-checking the release schedule, the cleaner LTS target is
actually Node 24:

- Node 24 (Krypton) is currently in Active LTS, supported until ~May 2028.
- Node 25 hit end-of-life on April 10 2026 — the floor merged in
  #7752 / #7749 / #7754 a day ago ships an already-EOL major.
- Node 26 was released May 5 2026 and does not enter Active LTS until
  October 2026.

So this PR reverts the Node 25 ratchet from those three PRs and lands
on Node 24 — Etherpad's runtime floor stays on a supported LTS for the
next ~2 years.

Runtime / infra
- `package.json` + `src/package.json`: `engines.node` `>=25.0.0` -> `>=24.0.0`
- `bin/functions.sh`, `bin/installer.sh`, `bin/installer.ps1`:
  `REQUIRED_NODE_MAJOR` 25 -> 24
- `Dockerfile`: `node:25-alpine` -> `node:24-alpine` (both stages).
  Corepack-via-npm workaround is intentionally kept: it works on
  Node 24 (which still ships corepack) and on Node 25+ (which doesn't),
  so the same recipe survives the next LTS bump without churn. Comments
  reworded accordingly.
- `snap/snapcraft.yaml`: pinned `NODE_VERSION` 25.9.0 -> 24.15.0; design
  notes + corepack comment adjusted
- `packaging/nfpm.yaml`: `nodejs (>= 25)` -> `nodejs (>= 24)` in
  top-level depends + deb/rpm overrides
- `packaging/bin/etherpad`: comment matches the new pin
- `packaging/README.md`: build prereqs + apt install snippet point at
  `node_24.x`; the long-stale "engines.node floor is 20" line is fixed
  while we're here
- `.github/workflows/*.yml`: setup-node `node-version` 25 -> 24 across
  every workflow; backend / frontend-admin / upgrade matrices
  `[25]` -> `[24]`
- `.github/workflows/deb-package.yml`: `NODE_MAJOR=25` + `node_25.x`
  smoke-test installer -> 24
- `bin/plugins/lib/npmpublish.yml`: 25 -> 24 (template propagates to
  the ~80 ether/* plugins via update-plugins workflow)

Docs
- `README.md`: install one-liner + Requirements -> Node.js >= 24
- `doc/npm-trusted-publishing.md`: runner requirement -> Node 24
- `doc/plugins.md` / `doc/plugins.adoc`: plugin metadata example
  `engines.node` -> `">=24.0.0"`

@types/node is left at ^25.8.0 — newer type definitions cover Node 24
runtime fine and avoid an unnecessary lockfile churn.

Companion homepage one-liner change to follow on ether/ether.github.com.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* docs(plugins): example engines.node = ">=22.0.0", not core's floor

Plugin code is overwhelmingly ace-hook glue and rarely uses Node-version-
specific APIs, so plugin engines.node should reflect the plugin's own
requirements, not track core. Showing core's 24-floor in the example
encouraged plugin authors to blindly copy a tighter pin than necessary
and locked plugins out of being installable on older Etherpad/Node
deployments. Use the most-recent Node LTS that has actually reached EOL
(20 -> EOL April 2026) as the example floor, i.e. >=22.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -9,12 +9,13 @@ ARG BUILD_ENV=git
 
 ARG PnpmVersion=11.0.6
 
-FROM node:25-alpine AS adminbuild
-# Node 25 no longer ships corepack at all, so install pnpm directly via
-# npm. The node:25-alpine image also bundles yarn; remove it first to
-# avoid leaving an unused binary on PATH. Drop bundled npm afterwards
-# — its older transitives (picomatch, brace-expansion) carry CVEs we
-# don't otherwise need.
+FROM node:24-alpine AS adminbuild
+# Install pnpm directly via npm (rather than via corepack) so the same
+# image recipe keeps working on Node 25+, where corepack has been
+# dropped from the distribution. The node:24-alpine image also bundles
+# yarn; remove it first to avoid leaving an unused binary on PATH.
+# Drop bundled npm afterwards — its older transitives (picomatch,
+# brace-expansion) carry CVEs we don't otherwise need.
 RUN rm -f /usr/local/bin/yarn /usr/local/bin/yarnpkg && \
     npm install -g pnpm@${PnpmVersion} && \
     rm -rf /usr/local/lib/node_modules/npm /usr/local/bin/npm /usr/local/bin/npx
@@ -24,7 +25,7 @@ RUN pnpm install
 RUN pnpm run build:ui
 
 
-FROM node:25-alpine AS build
+FROM node:24-alpine AS build
 LABEL maintainer="Etherpad team, https://github.com/ether/etherpad"
 
 # Set these arguments when building the image from behind a proxy
@@ -99,7 +100,8 @@ RUN groupadd --system ${EP_GID:+--gid "${EP_GID}" --non-unique} etherpad && \
 ARG EP_DIR=/opt/etherpad-lite
 RUN mkdir -p "${EP_DIR}" && chown etherpad:etherpad "${EP_DIR}"
 
-# Node 25 dropped corepack; install pnpm directly via npm, then drop
+# Install pnpm directly via npm (rather than via corepack) so the same
+# recipe stays valid on Node 25+, which dropped corepack. Then drop
 # both npm and the pre-bundled yarn binary to keep the runtime image
 # free of unused tooling and known-CVE transitives.
 #