scripts: check_maintainer_changes: Fix GitHub user access check

The PyGithub `Repository.get_collaborator_permission` function invokes the
GitHub REST API `/repos/{owner}/{repo}/collaborators/{username}/permission`
and returns the value of the `permission` attribute in the response.

As per the GitHub documentation [1], the `permission` attribute "provides
the legacy base roles of admin, write, read and none, where the maintain
role is mapped to write and the triage role is mapped to read", and this
caused the users with `triage` permission level (i.e. the users in the
`contributors` team) to be incorrectly flagged by the script.

This commit updates the script to use the `get_collaborator_role_name`
function, which correctly returns the up-to-date user permission/role name,
including `maintain` and `triage`.

Note that the `get_collaborator_role_name` function is only available in
PyGithub>=2.7.0.

[1] https://docs.github.com/en/rest/collaborators/collaborators?apiVersion=2022-11-28#get-repository-permissions-for-a-user



Signed-off-by: Stephanos Ioannidis <root@stephanos.io>