Use simpler and specific base images on CI

What does this Merge Request do and why?

Uses simpler and specific base images on CI instead of heavier one i.e. registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.7.2.patched-golang-1.14-git-2.29-lfs-2.9-chrome-87-node-14.15-yarn-1.22-postgresql-12-graphicsmagick-1.3.34.

This fixes an implicit regression of !1205 (merged). Changes the base container image for each job from registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.7.2.patched-golang-1.14-git-2.29-lfs-2.9-chrome-87-node-14.15-yarn-1.22-postgresql-12-graphicsmagick-1.3.34 to ruby:2.7 or golang:1.14. This MR does not update Go version used for checkmake.

Job name	before / sec	after / sec
checkmake	78	25
rspec	101	40
rubocop	91	40
verify-gdk-example-yml	80	25
yard	95	43

Each job reduces 50-55 sec by avoiding to download the huge container image 🚀 No performance degradation happens in all other jobs (code_quality, docs-lint, shellcheck).

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.