Reduce number of jobs in GitLab Pipeline

As pointed out by @stanhu on Slack (internal link)

Each pipeline launches 183 builds now (which means 183 clones of gitlab/gitlab-ee). Should we consider lowering this number? 132 of these are rspec jobs: 127 for pg9, 4 for pg10 58 are FOSS

Suggestions shared @ayufan

Can we use more changes: checks? With a fix of #31685 (closed) that is for 12.6, we could be more clever what we run, and use new rules: to provide more granularity on when to run given scopes.

@engwan

We recently split the migration specs.. maybe we could run them only when migrations change

@rymai

To reduce the number of jobs, I think we should actually delete tests.

Proposals

• Run qa-frontend-node:* jobs on master only - #42682 (closed)
• Reduce frequency of -as-if-foss jobs - #213944 (closed)
• Minimize the frequency of pg jobs to align with GitLab.com and Omnibus supported pg versions: - #215493 (comment 347639392)

What does success look like?

Reduction in number of jobs per pipeline without compromising quality gates.