Remove gem install bundler from Docker-based Ruby environments

What does this MR do?

Removes gem install bundler from Docker-based Ruby environments.

Follows up !24049 (merged).

Double check

$ git grep 'install bundler' |grep -v ^doc/update
bin/setup:  system! "gem install bundler --conservative"
bin/update:  system! 'gem install bundler --conservative'
doc/administration/monitoring/performance/grafana_configuration.md:gem install bundler
doc/ci/quick_start/README.md:  - gem install bundler --no-document
doc/ci/yaml/README.md:  - gem install bundler --no-document
doc/install/installation.md:    sudo gem install bundler --no-document

This MR is leaving bin/setup and bin/update as-is, as there is no prerequisite in these contexts.

All 4 other gem install bundler do not require users to use Docker:

doc/administration/monitoring/performance/grafana_configuration.md

=> https://gitlab.com/gitlab-org/gitlab-ce/blob/v11.6.0/doc/administration/monitoring/performance/grafana_configuration.md#L62

doc/ci/quick_start/README.md

=> https://gitlab.com/gitlab-org/gitlab-ce/blob/v11.6.0/doc/ci/quick_start/README.md#L80

doc/ci/yaml/README.md

=> https://gitlab.com/gitlab-org/gitlab-ce/blob/v11.6.0/doc/ci/yaml/README.md#L1645

doc/install/installation.md

=> https://gitlab.com/gitlab-org/gitlab-ce/blob/v11.6.0/doc/install/installation.md#L145

What are the relevant issue numbers?

Closes #55838 (closed)

cf. #55740 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• [n/a] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [n/a] Conforms to the database guides
• [n/a] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [n/a] Security reports checked/validated by reviewer