Enable placeholders for broadcast notifications

What does this MR do?

Part of #211369 (closed)

This removes the feature flag for broadcast placeholders and only enables placeholders for notifications. This feature flag changes the logic intentionally. During initial development it came apparent that placeholders only make sense for notifications, but this was then not reflected in the final MR.

This MR fixes this, removes the feature flag, adds a test for the new code path and cleans up the specs.

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