Enable `maven_packages_group_level_improvements` by default

☕ Context

We had some ~performance issues with the maven APIs. See #325869 (closed)

A first round of improvements have been implemented in !57600 (merged). The maven package registry is one of the most used one, so these improvements have been gated behind a feature flag .

Tracking issue: #326099 (closed)

The feature flag has been enabled for a few days on gitlab.com and here is the impact we saw: #325274 (comment 542069479).

🔍 What does this MR do?

• Enable maven_packages_group_level_improvements by default.

Why enabling it by default instead of removing it?

As described in the Context, the maven package registry is heavily used. We would like to deploy these improvements to self-managed users with a safety net to disable them.

🖼 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