Stage PHP 8.4 and latest promotion for Moodle 5.3 LTS

[erseco]

Switch default local/development and 5.3+ release builds to PHP 8.4/base 3.23; preserve PHP 8.3/base 3.20 for 4.5 and unsuffixed 5.0–5.2 releases. Restrict latest to the newest upstream 5.3 stable tag. Beta/RC and older versions cannot promote it.

Upstream has now released v5.3.0 and MOODLE_503_STABLE. The CI matrix and the persistent upgrade test are pinned to v5.3.0 (previously RC1).

Related: ateeducacion/moodle-playground#328 adds the same MOODLE_503_STABLE branch to the browser playground.

Validation (local, 2026-10-03, arm64, MOODLE_VERSION=v5.3.0, PHP 8.4, base 3.23)

• v5.3.0 + PostgreSQL 17: PASS (install, HTTP, Moosh, status checks incl. core_router, code sync, restart)
• v5.3.0 + MariaDB 11.4: PASS (same suite)
• v5.3.0 + SQLite (fork patch MDL-88218: Add experimental SQLite support for Moodle WASM environments (5.3) ateeducacion/moodle#7): PASS (same suite plus the SQLite DB file check)
• Persistent upgrade 4.5.14/PHP 8.3 → 5.3.0/PHP 8.4 on PostgreSQL 17: PASS (course, data file and custom plugin preserved; cron and restart)
• Shell unit suites (image-runtime, promote-latest, release-policy, sync-moodle-code, verify-moodle-version): PASS
• actionlint and git diff --check: PASS

The v5.3.0 environment.xml keeps the beta's minimums: PostgreSQL 17, MariaDB 11.4.0, MySQL 8.4.

Still pending

• MDL-88218: Add experimental SQLite support for Moodle WASM environments (5.3) ateeducacion/moodle#7 still targets the beta baseline. It applies to v5.3.0 as-is; retargeting it onto MOODLE_503_STABLE is optional.
• The v5.3.0 tag has not been synced or published yet. Merging this PR before the daily tag sync makes v5.3.0 build with PHP 8.4 and promote latest.
• Staging validation for real plugins/themes and a base-image security audit, as described in docs/moodle-53-promotion.md.

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Updated after merging #170 and #171: integrated the Hadolint fix and #173 serialized latest promotion. This draft now depends on #173; stable 5.3 selection changes the single shared policy, with promotion tests explicitly rejecting late 5.2 builds. All local shell suites and changed-script ShellCheck pass. Remains draft until Moodle 5.3 final and the documented release gates pass.

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Merged current main after #173 was merged and retargeted this draft to main. Added explicit beta1/beta2/rc2/rc10 regressions covering PHP selection, latest exclusion, exact release matching and rejection of stale prereleases. All local shell suites and changed-script ShellCheck pass (16 runtime, 15 release-policy, 19 version checks plus sync/promotion suites). docs/moodle-53-promotion.md records the release gate, prerelease handling and exact runtime mapping: 4.5 and unsuffixed 5.0–5.2 retain PHP 8.3; 5.3+ uses PHP 8.4. No promotion before final release tests and staging/backup validation.