CI pipeline fails due to outdated Drupal core version 10.4.1

Problem/Motivation

CI pipeline fails for ALL merge requests with the error:

Your requirements could not be resolved to an installable set of packages.

Problem 1
- Root composer.json requires drupal/core-recommended 10.4.1 -> satisfiable by drupal/core-recommended[10.4.1].
- drupal/core-recommended 10.4.1 requires drupal/core 10.4.1 but these were not loaded, because they are affected by security advisories ("SA-CORE-2025-001", "SA-CORE-2025-002", ...)

The `.gitlab-ci.yml` has hardcoded `CORE_STABLE: '10.4.1'` and `CORE_SECURITY: '10.4.1'` which are now blocked by Composer due to multiple security advisories.

Steps to reproduce

1. Create any merge request
2. CI pipeline fails at "Composer install and require drush" stage

Proposed resolution

Remove hardcoded `CORE_STABLE` and `CORE_SECURITY` variables from `.gitlab-ci.yml` to use default values from `gitlab_templates`. This is the recommended approach — upstream templates are automatically updated when new Drupal security releases are published.

Remaining tasks

- [x] Create patch
- [ ] Review
- [ ] Commit

User interface changes

None.

API changes

None.

Data model changes

None.