Add command to switch between current and unified SELinux policy

Description

We are moving toward consolidating our SELinux policy in Migrate existing GitLab SELinux policies into n... (#7453 - closed)

This will allow users to opt-in as we perform this change and move back if they discover issues.

Deliverables

• add gitlab-ctl sepolicy --version unified
• add gitlab-ctl sepolicy --version split
• ensure a system can switch between the policies that only one version is present at a time