feat: change owners to maintainers

[IronCore864]

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 pytest-interface-tester reflects the above changes.

See the other part of the change in the charm-relation-interfaces repo.

An extra note on the version: It has not been used since the last release, 3.1.0. So, although this PR technically is a breaking change, it doesn’t break anything. That’s why I only slightly bumped the version to 3.1.1, not strictly following the SEMVER rules.

[PietroPasotti]

LGTM, modulo a minor consistency nit

[IronCore864]

Refactored, please go ahead and merge. Thanks!