Draft: Remove implicit licensed feature flags and rework `beta_*`

What does this MR do?

Related to: #259611 (closed)

This MR removes implicit licensed feature flags, instead replaces it with an explicit and required ones, connected with type: :licensed.

This does:

• Removes implicit check from feature_available?
• Adds push_frontend_beta_feature_available
• Changes beta_feature_available?(feature, default_enabled: false) to explicitly check for feature flag
• Updates all places to adhere to new
• Adds all licensed features that seems to be gated behind beta_feature_available?
• Removes unused multiple_merge_request_assignees which is default_enabled: true anyway
• Removes alpha_feature_available?
• Updates documentation

Screenshots

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

• 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