Update Ruby 2.6 to 2.7 in www-gitlab-com image

www-gitlab-com image is updated as follows:

• Updates Ruby 2.6 to 2.7
• Drops Ruby 2.4 test and build (orphan from !198 (merged))
• Updates the base container image from stretch (ruby:2.6.6-slim-stretch) to buster (ruby:2.7.2-slim)
  • which causes deb package conflicts between buster (stable) and bullseye (testing) cf. failed test https://gitlab.com/gitlab-org/gitlab-build-images/-/jobs/991500535
• Updates pyyaml from 5.3.1-3 to 5.4.1
• Pins yamllint from 1.25.0-1 via Debian package to 1.25.0 via pip in order to drop a hack in !332 (merged)
• Pins pathspec from 0.8.1-1 via Debian package to 0.8.1 via pip
• Introduces multi-stage build to utilize google-cloud-sdk via pre-built by Google Cloud to simplify complicated scripts introduced in !295 (merged)
• Drops python-crcmod (for Python 2) package while python2 package is still installed via dependncies from texlive-xetex package.
  • Note: python-crcmod and python3-crcmod are only used via google-cloud-sdk
  • Note 2: texlive-xetex, introduced in e305ced6 in 2016-07-21, requires/depends on python2, but not sure why we need it