Sync only active release managers to instance groups

We run a release managers sync task which does a couple of things:

1. Queries release/manager related groups on three instances we operate on
2. Checks against config/release-managers.yaml to confirm if the people in groups are authorised to perform RM operations
3. Adds/removes to groups based on the outcome of the check

This MR looks to achieve the following:

1. Fetch the latest active release managers based on the date, and the schedule on about.gitlab.com/community/release-managers
2. Sync access to the release-managers group across instances for active release managers
3. Removing anyone who is not scheduled to perform those duties

This will allow us to remove the need for people manually pinging RM's in the issues/MRs when needed and always reference the RM group knowing they will reach currently active release managers.