Enable achievements feature flag by default

What does this MR do?

This MR changes the default value of the achievements feature flag from false to true, enabling the achievements feature for all users by default.

Related issues

Closes #386817

Author's checklist

• Feature flag configuration updated
• Default value changed from false to true

Review checklist

• Confirm that the achievements feature is ready for general availability
• Verify that enabling this by default won't cause performance issues
• Check that all related documentation is up to date