Switch to custom 'committers' role for new repos

This switches the default repository rights for committers from 'push' to 'committers', which is a custom role that grants the same rights as 'push' (write) plus the ability to create protected tags when tag rules are in place. See https://issues.apache.org/jira/browse/INFRA-24644 for additional context on this role.
diff --git a/server/plugins/github.py b/server/plugins/github.py
index e6501a4..0e9088f 100644
--- a/server/plugins/github.py
+++ b/server/plugins/github.py
@@ -557,9 +557,9 @@
         await self.org.api_delete(url)
 
     async def add_repository(self, reponame: str):
-        """Adds a single repository to the team"""
+        """Adds a single repository to the team with the custom 'committers' role (write access plus can create protected tags)"""
         assert (
             self.org.orgid
         ), "Parent GitHubOrganization needs a call to .get_id() prior to membership updates!"
         url = f"https://api.github.com/organizations/{self.org.orgid}/team/{self.id}/repos/{self.org.login}/{reponame}"
-        await self.org.api_put(url, {'permission': 'push'})
+        await self.org.api_put(url, {'permission': 'committers'})