chore: update dev deps to phpstan 2.x, PHP 8.0+, WP 6.6-6.9 (v1.2.0)

[gin0115]

• composer.json: require PHP 8.0+, phpstan 2.x, phpstan-wordpress 2.x, phpunit 8|9, WP 6.9 test deps. Drop phpcompatibility, roave/security-advisories and the explicit squizlabs pin (installed transitively via the phpcs installer).
• Retire WP_6_2 / WP_6_3 / WP_6_4 / WP_6_5 workflows. Refresh WP_6_6 / WP_6_7 and add WP_6_8 / WP_6_9. All run PHP 8.0-8.4 against mysql:8.4. WP_6_9 uploads coverage via codecov/codecov-action@v4 under ${{ secrets.HTTP_CODCOV }}.
• Add standard .scrutinizer.yml block.
• phpcs.xml: add the curl-alternative excludes and PSR12.Files.FileHeader.IncorrectOrder exclude; keep the short-ternary excludes.
• tests/wp-config.php: suppress WP 6.8 wp_is_block_theme early-call E_USER_NOTICE under PHPUnit's notice-to-exception conversion; switch the CI DB name from the stale 'bp_events' over to 'pc_core_tests'.
• tests/.env (gitignored, already in .gitignore) added for local dev.
• Docblocks: fill in descriptions on every bare @param / @return / @throws across src/ (HTTP.php + HTTP_Helper.php).
• README: prune stale WP 6.2-6.5 badges, add WP 6.6-6.9, add 'Tested Against' block, 1.2.0 changelog entry.

20 tests, 68 assertions green. phpstan level 8 clean. phpcs clean.