Update rails project template to rails 6.1

What does this MR do?

Vendors the changes from gitlab-org/project-templates/rails!21 (merged)

See #326177 (closed)

Screenshots (strongly suggested)

In my GDK, created a project from this template and ran an Auto DevOps pipeline with a Kubernetes cluster. Everything works, except code_quality due to ruby 3.0, but this job is allowed to fail so this is definitely an improvement over a fully failing pipeline.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.