ci: run the test matrix on the runner natively, not in a Debian container

[dkastl]

Motivation

Every matrix job pulled a ruby:*-bullseye image and then apt-installed build tools, Chrome, and Node (via NodeSource) from deb.debian.org. Those downloads were the source of the recurring Error reading from server - read (104: Connection reset by peer) failures that flaked random single matrix jobs across the last several PRs (#383, #385, #386, #388), each needing a manual re-run.

Change

Switch the test job to the standard runner-native GitHub Actions pattern, which removes almost all of that download surface:

• Drop container: ruby:*-bullseye; run directly on ubuntu-latest.
• ruby/setup-ruby@v1 for the matrix Ruby (3.3 / 3.4) — prebuilt binaries, no apt.
• actions/setup-node@v6 for Node — drops the NodeSource apt step (already used by the frontend job).
• apt only for native-gem headers (libpq-dev for pg, libgeos-dev for rgeo-geos) from Ubuntu's Azure-hosted mirrors, which are far more reliable on GH runners than deb.debian.org. gcc/make/patch and Chrome (for the system test) are preinstalled on the runner.
• DB host → 127.0.0.1: the postgis service is reachable by service name only from inside a job container; on the runner host it's published on localhost via the existing ports: 5432:5432 mapping. (This is the one functional gotcha of dropping the container.)

Tradeoffs

• The host OS for the Ruby side becomes Ubuntu 24.04 instead of Debian 11. This does not affect plugin-level tests (Ruby + Redmine + PostGIS) — native gems just compile against the runner's libraries. It's the pattern most Redmine-plugin CIs use.
• The only thing the container provided was an OS close to a deployment image, which isn't a goal for plugin CI and isn't worth the recurring flakes.

No change to coverage: same Redmine/Ruby/PostGIS matrix, same unit/functional/integration/system suites, same uninstall test.

[Copilot]

Pull request overview

Updates the PostGIS CI workflow to run the Ruby/Redmine/PostGIS test matrix directly on the GitHub-hosted Ubuntu runner (instead of inside a Debian-based Ruby container) to reduce network flakiness and simplify dependencies.

Changes:

• Removes the container: ruby:*‑bullseye configuration and installs Ruby via ruby/setup-ruby@v1.
• Sets up Node via actions/setup-node@v6 and limits apt installs to native gem headers (libpq-dev, libgeos-dev).
• Switches the DB host from the service name to 127.0.0.1 to match runner-native service port publishing.

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