Howto change default branch for a repository by CLI or API

Problem to solve

We need to have a bash script that changes the default branch of a repository. I connot find a git CLI command or API operation which could do that

• I could not find an operation that would do the job. Manually using the UI seems to be the only option.
• I would expect a API or CLI command to do that job

Steps to reproduce

• Using gitlab UI create a repository, define a branch (master), define it as the default branch.
• Using gitlab UI create a second branch (newmaster)
• use (unix) shell command to change the default branch to “newmaster” (how???)
  -OR-
• use curl API command to change the default branch to “newmaster” (how???)

Versions

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• Self-managed
• GitLab.com SaaS

Versions

• GitLab Community Edition [v17.0.2]
  (Maintenance Rake tasks | GitLab)):

The projects API endpoint supports the create action and edit action, which can pass the default_branch parameter.