feat: use team id as maintainers

[IronCore864]

This PR changes the previous "owners", a list of GitHub user IDs, to "maintainers", a string of GitHub team ID. Assigning issues to a team is not supported, so an extra feature is added where user IDs are fetched from team ID.

More background:

Per discussions in this issue (see comment here and here) and an email thread, we have agreed to the followings:

• Use "maintainers" instead of "owners", because "owners" indicates ownership while the interface is meant to be "owned" by a group around it.
• Put a GitHub team ID as "maintainers", instead of a list of users. This eliminates the overhead of maintaining the list of user IDs. The purpose of this field is to assign issues to users when a certain interface test fails; although it's not supported to assign an issue to a team, this can be worked out in the charm-relation-interfaces repo's run_matrix.py script.

This PR to charm-relation-interfaces reflects the above changes, see the other part in the pytest-interface-tester repo. Before merging into the main branch, it can’t be tested using GitHub Actions, but I’ve tested it in a forked repo in my own org, see here.

[IronCore864]

Now that the PR for pytest-interface-tester is merged, this PR can be merged to use the latest pytest-interface-tester, v3.1.1, that has the "maintainer" feature.

Note that this PR needs to be merged first in order for the checks to pass.

[benhoyt]

Looks good. One code style suggestion.

[IronCore864]

This PR can be merged since pytest-interface-tester 3.2.0 has been released with the maintainer feature, and the tests have been fixed, see passed checks below.

Squash all commits, sign, and merge now.