Update the GitLab maintenance policy

Context

To accommodate for the extension of the maintenance policy, we should update our maintenance policy to reflect the new policy.

Maintenance policy docs

• https://docs.gitlab.com/policy/maintenance/
  • Where there is differentiation between the versions supported for security vs bug fixes, we should modify it so that both security and bug fixes get backported to the previous two monthly releases in addition to the current stable release
• https://docs.gitlab.com/policy/maintenance/#backporting-to-older-releases
  • This section no longer applies to backporting bug fixes to more than one stable release.
  • "older releases" should now indicate older than previous two monthly releases.

Exit Criteria

• The maintenance policy docs should reflect the new maintenance policy with the extension.
• Merge requests should be reviewed, but not merged until the extension is rolled out