feat: Enable discard-approval-on-plan for gitlab

[moritz31]

what

This pull request enables discard-approval-on-plan for gitlab.

why

This enables a use case for me, which is already covered by atlantis but only for github.

tests

Adding a test with testcases in gitlab_client_test.go TestGithubClient_DiscardReviews.
Running the stack locally and testing from gitlab that the pull request approval is revoked.

references

Gitlab API Documentation https://docs.gitlab.com/api/merge_request_approvals/#reset-approvals-of-a-merge-request

[X-Guardian]

Hi @moritz31, thanks for raising this PR.

Can you update the documentation at https://github.com/runatlantis/atlantis/blob/main/runatlantis.io/docs/server-configuration.md#--discard-approval-on-plan also detailing the GitLab token requirements.

Please also update the PR title and description to say that this change is enabling the discard-approval-on-plan feature for GitLab (The PR title is used for the Atlantis changelog entry)

[moritz31]

Hope this now fits, need to make some bigger adjustments because of no logger passed to that method.

[X-Guardian]

Looking good @moritz31. The tests are failing if you could take a look.

[X-Guardian]

Thanks for this @moritz31

[X-Guardian]

Can you test that this is working correctly for you using one of these dev container images:

• dev-alpine-7b40007
• dev-debian-7b40007

[moritz31]

Thanks for your guidance! I have tested the alpine container in our live environment and it works like expected