skip e2e:test-on-omnibus messages for docs and ci changes in backport MRs

Context

There are automated messages on backport MRs that guides to check the e2e:test-on-omnibus downstream pipeline.

This is unnecessary when the MR is for documentation and CI changes.

Danger behaviour changes

This issue is to remove that message for backport MRs that only contain documentation, CI, or spec changes.

• &1579 (comment 2647234715)
• Slack link (internal only)
• gitlab-org/gitlab!199460 (comment 2654653078)

We should also update the Danger logic not to fail with the e2e error message if this is the case gitlab-org/gitlab!199222 (comment 2653079851)

Exit Criteria

• For MRs that only contain documentation, CI, or spec changes, there shouldn't be a message to check the e2e:test-on-omnibus pipeline
• For those MRs, the danger logic shouldn't fail either.
• Backport the MRs to maintained branches