Upgrade the Ruby on Rails version to 6.1

This issue is to track upgrading Rails version to 6.1 in https://gitlab.com/gitlab-org/gitlab project.

Confidence level: 50%

Release Notes

In GitLab 14.0 release, we upgraded Ruby on Rails version to 6.1 to take advantage of the latest improvements of the application framework. Please refer to Ruby on Rails release note for more details, https://guides.rubyonrails.org/6_1_release_notes.html.