Accept maintainer_note parameter in REST runner registration

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR adds support for taking in a maintainer_note field in the runner registration endpoint. A gitlab/gitlab-runner MR will soon take advantage from this addition so as to allow an admin to specify notes to be associated with the new runner.

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Part of #348299 (closed)

doc/architecture/blueprints/ci_data_decay/index.md

@@ -15,8 +15,7 @@ the CI/CD subsystem has evolved significantly. It was [integrated into GitLab in
 and has become [one of the most beloved CI/CD solutions](https://about.gitlab.com/blog/2017/09/27/gitlab-leader-continuous-integration-forrester-wave/).
 
 On February 1st, 2021, a billionth CI/CD build was created and the number of
-builds is growing exponentially. We expect to see 20M builds created daily on
-GitLab.com in late 2024.
+builds [is growing exponentially](https://docs.gitlab.com/ee/architecture/blueprints/ci_scale/).
 
 GitLab CI/CD has come a long way since the initial release, but the design of
 the data storage for pipeline builds remains almost the same since 2012. In