MNTOR-4499: drop fk for feature flag events table (option 2)

[mansaj]

References:

Jira: MNTOR-4499

Description

Option B: if we want to completely abolish the foreign key in feature flag events table. See slack thread for clarification

Screenshot (if applicable)

Not applicable.

How to test

Checklist (Definition of Done)

• Localization strings (if needed) have been added.
• Commits in this PR are minimal and have descriptive commit messages.
• I've added or updated the relevant sections in readme and/or code comments
• I've added a unit test to test for potential regressions of this bug.
• If this PR implements a feature flag or experimentation, I've checked that it still works with the flag both on, and with the flag off.
• If this PR implements a feature flag or experimentation, the Ship Behind Feature Flag status in Jira has been set
• Product Owner accepted the User Story (demo of functionality completed) or waived the privilege.
• All acceptance criteria are met.
• Jira ticket has been updated (if needed) to match changes made during the development process.
• Jira ticket has been updated (if needed) with suggestions for QA when this PR is deployed to stage.

[github-actions]

Preview URL 🚀 : https://blurts-server-pr-5888-mgjlpikfea-uk.a.run.app

[Vinnl]

Linking option 3, which I think is the safer option since it will keep it mandatory for feature flag changes to have an author associated: #5905