CI: Use bundler's deployment mode

What does this MR do and why?

This simplifies the CI and simplecov setup because:

• Gemfile.lock is frozen (similar to BUNDLE_FROZEN=true)
• Bundle path is set to vendor/bundle
• Simplecov filters vendor/bundle by default

See:

• !208 (comment 1721350110)
• https://bundler.io/v2.4/man/bundle-install.1.html#DEPLOYMENT-MODE
• https://bundler.io/v2.4/man/bundle-install.1.html#REMEMBERED-OPTIONS
• https://bundler.io/v2.1/whats_new.html#cli-deprecations

Verification on CI

• Coverage reports 100% (663 / 663 LOC) via https://gitlab.com/gitlab-org/ruby/gems/gitlab-styles/-/jobs/5900215299