Require node 22 to match terriajs-server 5 alpha

[main-tech]

Problem

The Dockerfile builds on node:20, but
terriajs-server@5.0.0-alpha.2 (already pinned in dependencies) requires node >=22. A
clean docker build therefore fails at yarn install:

[build 4/5] RUN yarn 
  install --network-timeout 1000000:
[1/5] Validating package.json...
[2/5] Resolving 
  packages...
[3/5] Fetching packages...
error terriajs-server@5.0.0-alpha.2: The engine 
  "node" is incompatible with this module. Expected version ">=22.0.0". Got "20.20.2"
error
  Found incompatible module.
ERROR: failed to build: process "/bin/sh -c yarn install 
  --network-timeout 1000000" did not complete successfully: exit code: 
  1

engines.node was also still >= 20.0.0, contradicting the existing
CHANGES.md note that 0.4.7 "Requires node version 22 or above".

Fix

• Bump all
  Dockerfile stages (develop, build, deploy) from node:20/node:20-slim to
  node:22/node:22-slim
• Bump engines.node to >= 22.0.0
• Note the bump in
  CHANGES.md

Verified: docker build now completes successfully end-to-end (yarn install + gulp release) on node 22.