Turn database testing job into manual action

What does this MR do?

We currently have the actual testing job on the downstream pipeline set to manual. This allows the pipeline to be prepared automatically, but needs a maintainer to go in an play the testing job each time.

With this change, I would like to

1. turn the db:gitlabcom-database-testing job into a manual action (here)
2. and follow-up with removing the when: manual part from the downstream testing job (gitlab-org/database-team/gitlab-com-database-testing!18 (merged))

This will allow anybody in this project to kick of automated testing by playing the db:gitlabcom-database-testing job.

Eventually this should become fully automated, so we don't need when: manual in this job anymore. However, I would like to only gradually open this up - so we can see how the testing system behaves with more load.