Use a default branch that's not "master" in template

What does this MR do?

The Git community has since some time started to reconsider the naming of repository branches that might bring negative connotations. As an example, GitHub has announced that the default name for repo is now main.

The MR aims to solve a practical problem when a repository wants to adhere to this value and practice. As an example, an OSS project I am maintaining and for which I started playing with GitLab managed Terraform State. See here where I need to extend the job only in order to use a different default branch.

This initiative fits with or Values of DIB and guidelines in communication (e.g. use of whitelist/blacklist) and aligns with plans related to other features to support the same approach (as per this conversation (internal Slack thread)).

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because there are no visual changes.
• 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