Raise minimum PHP to 8.2 and expand CI matrix (XWPENG-46)

[shadyvb]

Summary

• Breaking change (5.0.0): Minimum PHP is 8.2 (composer.json ^8.2, Requires PHP: 8.2, PHPCS testVersion 8.2-). Sites below 8.2 get a capability-gated admin_notices / network_admin_notices message; Stream does not bootstrap (no fatal, no shutdown echo into REST/AJAX/feeds/CLI).
• CI: Lint/Test and E2E run on a PHP 8.2 / 8.3 / 8.4 matrix with per-version Composer cache keys, WP_ENV_PHP_VERSION, and composer check-platform-reqs after install. Deploy workflows use PHP 8.2.
• Tests / hygiene: Multisite WP_TEST_ACTIVATED_PLUGINS applied via site_option_active_sitewide_plugins map (fixes Jetpack path noise); testcase silences post-bootstrap header/cookie warnings and Yoast indexable watchers on factory blogs; PHPCS/doc tweaks and Admin_Test stub fix; removed obsolete PHP 5.3.6 backtrace guard in Log. Host unit coverage in Fail_Php_Version_Unit_Test (hook registration + capability-gated notice output).

Stack context

Stacks on #1977 (ticket/XWPENG-45-wp-env) → part of GitHub stack #1976 (develop ← … ← this PR).

Test plan

• composer check-platform-reqs on PHP 8.2+
• composer test-unit
• npm run lint
• npm run test (wp-env, single-site + multisite)
• npm run test-e2e (at least one matrix leg in CI)
• Fail_Php_Version_Unit_Test (admin notice hooks + activate_plugins gate)

Branch protection migration (maintainers)

CI job display names changed from static Lint and Test / E2E to matrix names. Update required status checks on develop / master before or right after merge:

1. Lint and Test (PHP 8.2)
2. Lint and Test (PHP 8.3)
3. Lint and Test (PHP 8.4)
4. E2E (PHP 8.2)
5. E2E (PHP 8.3)
6. E2E (PHP 8.4)

Remove obsolete Lint and Test and E2E entries. Keeping only the old names blocks every PR; removing them without adding the six allows merges without CI.

Review artifacts

Local review notes under .ai/reviews/ (gitignored): Stage 4 applied admin-notice fix (S2-H1), version/changelog 5.0.0 (S2-H2), platform-reqs in CI (S2-M1), and documented branch-protection migration (S2-H3).