[BUG] Pagination prevent fetching more than 100 repositories

[GradedJestRisk]

Current perPage limit is 100 in Gitlab, and the script use 9999, thus fetching no more than 100 repositories.

https://docs.gitlab.com/ee/api/rest/index.html#pagination

per_page Number of items to list per page (default: 20, max: 100).

https://github.com/adroste/gitlab-clone-group/blob/main/gitlab-clone-group.py#L17

api_url = 'https://' + posixpath.join(args.gitlab_domain, 'api/v4/groups/', args.group_id, 'projects') + '?per_page=9999&page=1&include_subgroups=true'

[adroste]

I personally never hit this limitation but I'm happy to accept a PR.
Should easily be doable by iterating over the pages ?per_page=100&page=n and concatenating the responses.