Remove the `maven_packages_group_level_improvements` feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

🍞 Context

The maven_packages_group_level_improvements feature flag has been enabled by default in %13.11 with !59136 (merged).

Time to cleanup the feature flag and code.

See the related rollout issue: #326099 (closed)

🔎 What does this MR do?

• Remove the maven_packages_group_level_improvements feature flag

📷 Screenshots (strongly suggested)

n / a

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because ________.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team