BED-4934: Posture page feature flag

[maffkipp]

Description

Adds a new feature flag to the current migration file with the key updated_posture_page. The flag should initially be set to false and should be non user-updatable.

Motivation and Context

This feature flag will be used for toggling to the new posture page during development.

This PR addresses: BED-4934

How Has This Been Tested?

Confirmed migration runs on startup and creates the new flag with the expected settings and initial value.

Screenshots (optional):

Types of changes

• Chore (a change that does not modify the application functionality)
• Database Migrations

Checklist:

• I have met the contributing prerequisites
  • Assigned myself to this PR
  • Added the appropriate labels
  • Associated an issue: Issues and Pull Requests README #672
  • Read the Contributing guide: https:/SpecterOps/BloodHound/wiki/Contributing
• I have ensured that related documentation is up-to-date
  • Open API docs
  • Code comments (GoDocs / JSDocs)
• I have followed proper test practices
  • Added/updated tests to cover my changes
  • All new and existing tests passed

[specter-flq]

Makes sense imo, but will defer to the veterans for approval.

[byt3exec]

Makes sense for DM.sql

[elikmiller]

Makes sense for DM.sql

@byt3exec perhaps you meant to comment on a different PR?