Resolve "Need a way to ensure all newly created paid groups are indexed"

What does this MR do?

Issue #220246 (closed)

• Create a new feature flag :elasticsearch_index_only_paid_groups defaulted to OFF
• Use new flag in checks for whether to index namespace after GitlabSubscription created when subscription is created or updated. Subscription plan must be: hosted, paid, not a trial

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Added unit tests for multiple scenarios

• Local Customers Portal manual tested scenarios:

  • feature flag disabled
    • new paid bronze subscription --> ElasticsearchIndexedNamespace record not created
  • feature flag enabled
    • new paid bronze subscription --> ElasticsearchIndexedNamespace record created
    • upgrade existing indexed bronze subscription to silver --> ElasticsearchIndexedNamespace record not created
    • upgrade existing not indexed bronze subscription to silver --> ElasticsearchIndexedNamespace record created
    • new self managed silver subscription --> ElasticsearchIndexedNamespace record created

• 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