Drupal 12 compatibility

Problem/Motivation

Drupal 12 is on the horizon. To ensure a smooth transition for Commerce
sites when it releases, it is worth starting to prepare the codebase now.

Currently, installing Commerce alongside a Drupal 12 codebase is not
possible because:

• composer.json declares "drupal/core": "^10.3 || ^11",
  which excludes Drupal 12.
• All sub-modules declare core_version_requirement: ^10.3 || ^11
  in their .info.yml files.

Proposed resolution

Update the version constraints across the project:

• composer.json: "drupal/core": "^10.3 || ^11 || ^12"
• All .info.yml files:
  core_version_requirement: ^10.3 || ^11 || ^12

A preliminary code audit shows no usage of APIs deprecated in Drupal 10
and removed in Drupal 12. Hook implementations already use OOP
#[Hook] attributes. The main remaining risk is upstream
dependencies:

• drupal/address ^2.0
• drupal/entity ^1.0
• drupal/profile ^1.2
• drupal/state_machine ^1.5

These packages will also need to declare Drupal 12 compatibility before
Commerce can be installed without conflicts.

Adding OPT_IN_TEST_NEXT_MAJOR: '1' to the Commerce
.gitlab-ci.yml would provide an automatic signal: once all
upstream dependencies declare Drupal 12 compatibility, the job will turn
green without any manual follow-up needed.

Additional context

Contrib modules that depend on Commerce are
currently unable to run Drupal 12 CI jobs because of this constraint.
The _LENIENT_ALLOW_LIST workaround available in the Drupal
GitLab CI templates does not resolve transitive symfony/yaml
version conflicts introduced by the current drupal/core constraint.