Add feature flag for alert deprecation [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Adds a feature flag managed_alerts_deprecation for the alert deprecation rollout. This is to allow the deprecation to be activated on June 7th.

This flag will be used for the various features that need to be deprecated together, as described in this parent epic: &5877.

Adds a usage of this feature flag, to satisfy the build job that requires at least one feature flag reference in the code. This will eventually be used anyway so it's not big deal to add it here 🙂

Rollout issue: #331863 (closed)

Screenshots (strongly suggested)

no visual change

Does this MR meet the acceptance criteria?

Conformity

• [n/a] I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• [n/a] I have added/updated documentation, or it's not needed. (Is documentation required?)
• [n/a] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [n/a] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [n/a] I have tested this MR in all supported browsers, or it's not needed.
• [n/a] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [n/a] Label as security and @ mention @gitlab-com/gl-security/appsec
• [n/a] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [n/a] Security reports checked/validated by a reviewer from the AppSec team