Rename the `Master` role to `Maintainer`

Requirements

• Rename Master role to Maintainer.
• Replace the copy below with Allow commits from members who can merge to the target branch.

Original description

Summary

The administrators of a repository are labelled “Master”, which is problematic language, both because of it being gendered, and because of the slavery-related connotations. Alternatives could be “custodian”, “administrator”, ...

Steps to reproduce

Create a repository, be labelled its master.

Example Project

What is the current bug behavior?

Repository administrators are called “masters”.

What is the expected correct behavior?

Use alternative wording.

Output of checks

This bug happens on salsa.debian.org, and likely on gitlab.com too

Possible fixes

Use alternative wording, such as “custodian” or “administrator”.