Draft: Use specific production branch + Merge request pipeline compatible

What does this MR do?

CI_DEFAULT_BRANCH

Replace direct "master" reference in AutoDevOps jobs by $CI_DEFAULT_BRANCH * Allow a "production" branch to be used as default one * https://docs.gitlab.com/ee/user/project/repository/branches/#default-branch

Merge request pipeline

Enable Merge Request flow in AutoDevOps as already documented

Fix #219913

Work explanation

rules:
  - if: '$CI_COMMIT_BRANCH || ( $CI_MERGE_REQUEST_IID && $CI_WORKFLOW_MR )'

I use a CI_WORKFLOW_MR variable set to true in workflow to prevent duplicate pipeline for project using neither of proposed workflow.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team