Run predictive tests with single-db setup in child pipelines

What does this MR do and why?

• Don't run all single-db jobs in predictive pipelines
• Run the predictive counterparts for the single-db jobs instead
• After approval, things should stay the same that we run all single-db jobs when the conditions meet

Details

Run predictive tests with single-db setup

• rspec:predictive: https://gitlab.com/gitlab-org/gitlab/-/pipelines/1268172683
• rspec:predictive single-db: https://gitlab.com/gitlab-org/gitlab/-/pipelines/1268172578
• rspec:predictive single-db-ci-connection: https://gitlab.com/gitlab-org/gitlab/-/pipelines/1268172706

All of them should be running the same tests, just with different setup. And EE ones:

• rspec-ee:predictive (name updated in the next pipeline): https://gitlab.com/gitlab-org/gitlab/-/pipelines/1268172592
• rspec-ee:predictive single-db: https://gitlab.com/gitlab-org/gitlab/-/pipelines/1268172597
• rspec-ee:predictive single-db-ci-connection: https://gitlab.com/gitlab-org/gitlab/-/pipelines/1268172594

All of them should be running the same tests, too.

MR acceptance checklist

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #453973 (closed)