feat: #3616461 Add support for Drupal 11 with ^11, plus basic testing and linting

Thanks Norbert (nicothezulu) for reporting this in #3616461, and for the version-bump commit this MR builds on.

Problem/Motivation

Varbase Theme (Bootstrap 3 - LESS) 6.x declared core_version_requirement: ~8 || ~9 || ~10 and had no CI pipeline. It needs Drupal 11 support, plus basic automated testing and linting so future changes are gated.

Proposed resolution

• core_version_requirement: ^10 || ^11
• drupal/core: ^10 || ^11
• Bump drupal/bootstrap to ^3.40 — the Drupal-11-capable release of the Bootstrap 3 base theme ("Works with Drupal ^10.4 || ^11")
• Add GitLab CI (drupal/gitlab_templates) with composer-lint, cspell, phpcs, phpstan as blocking validate jobs (eslint/stylelint skipped — vendored Bootstrap 3 JS and CSS compiled from LESS, not hand-authored)
• Add a slim .gitlab-ci-local.yml, .phpcs.xml, .cspell.json, phpstan.neon
• Fixed two typos found along the way: a Boostrap comment, and foter_classes → footer_classes

Verification/Testing

• Local gitlab-ci-local slim pipeline is GREEN: composer-lint, cspell, phpcs, phpstan all PASS.
• Installed on a fresh Drupal 11.4.5 DDEV site with drupal/bootstrap:3.40.0: theme enables cleanly, set as default, front page and /user/login return HTTP 200, Bootstrap 3 markup renders correctly, zero PHP errors/warnings (Playwright-verified).

AI-Generated: Yes

Checkpoints:

• File an issue
• Addition/Change/Update/Fix
• Testing to ensure no regression
• Automated unit testing coverage
• Automated functional testing coverage
• UX/UI designer responsibilities
• Readability
• Accessibility
• Performance
• Security
• Developer Documentation
• User Guide Documentation
• Reviewed by human
• Code review by maintainers
• Full testing and approval
• Credit contributors
• Review with the product owner
• Release notes snippet
• Release