Introduce dedicated rules for rspec jobs per level

What does this MR do?

Also, run unit and integration jobs only when there are backend changes.

Verification matrix

• MR without backend changes: !34532 (closed). Unit and integration jobs aren't created: https://gitlab.com/gitlab-org/gitlab/-/pipelines/156414436/builds
• MR with backend changes: !34533 (closed). Unit and integration jobs are created: https://gitlab.com/gitlab-org/gitlab/-/pipelines/156414392/builds

Closes #24339 (closed).