Changelog for v3.12.0

See the v3.12.0 milestone

Most prominently we missing:

• https://github.com/gitlabhq/terraform-provider-gitlab/pull/916
• https://github.com/gitlabhq/terraform-provider-gitlab/pull/910
• https://github.com/gitlabhq/terraform-provider-gitlab/pull/891

I consider #891 to be optional, but if possible, let's release it, too ;)