Add support for Drupal 11 with ^11

Problem/Motivation

Vartheme (Bootstrap 3 - LESS) 6.x declared Drupal core ~8 || ~9 || ~10 and shipped no CI, so it does not install on Drupal 11 and has no testing/linting.

Steps to reproduce

1. On a Drupal 11 site, require the theme: composer require drupal/vartheme.
2. Try to enable Vartheme.
3. It is blocked by the core compatibility check.

Proposed resolution

• Set core_version_requirement: ^10 || ^11 and drupal/core: ^10 || ^11.
• Require drupal/bootstrap:^3.40 (the Drupal 11 compatible Bootstrap 3 base theme).
• Add GitLab CI (drupalci templates): composer-lint, cspell, phpcs, phpstan — plus .gitlab-ci-local.yml, .phpcs.xml, .cspell.json, phpstan.neon.

Remaining tasks

• ✅ 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

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Adds Drupal 11 support (^10 || ^11, drupal/bootstrap ^3.40) and basic CI testing/linting.