ci: run dependency and license scanner on MRs only if go.mod changed

What does this MR do?

Dependency scanners and license scanning in particular take up the majority of CI usage in Pages (see pipeline for latest commit: https://gitlab.com/gitlab-org/gitlab-pages/-/pipelines/556342912/builds).

This MR changes gemnasium dependency scanner and license scanning for MRs to only run if go.mod changed.

Note: dependency scanners run regardless of go.mod for full pipelines (commit/merge).

This is part of an effort to reduce CI minutes consumption in GitLab products in response to the recent change in CI/CD minutes usage for contributors.

TODO

• I added the Changelog trailer (e.g. Changelog: added) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines