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[bot]]

Preview URL :rocket: : 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: https://github.com/mozilla/blurts-server/pull/5905