Remove global _custom_access registration from Easy Encryption admin access checker

Problem/Motivation

The PrivateKeyStorageMigratorAccessCheck service was registered globally with:

- { name: access_check, applies_to: _custom_access } 

This causes the access checker to be executed for all routes using Drupal's _custom_access requirement, even when those routes are unrelated to Easy Encryption.

As a result, unrelated routes such as render endpoints can fail because the custom access checker is incorrectly invoked outside its intended scope.

This creates issues such as:

• Broken render endpoints
• Unexpected access validation behavior
• Route handling conflicts with unrelated modules
• Difficult debugging due to globally applied access logic

Proposed resolution

Remove the global access checker registration:

tags: - { name: access_check, applies_to: _custom_access } 

Keep the service available only for explicit usage where needed instead of automatically applying it to every route that uses _custom_access.

Benefits

• Prevents unintended access checks on unrelated routes
• Fixes render endpoint failures
• Improves route isolation
• Makes access control behavior more predictable

Remaining tasks

User interface changes

API changes

Data model changes