Switch triagebot team to maintain role

[ehuss]

This upgrades the triagebot team's permission role from write to maintain so that they are able to merge PRs. GitHub's default branch protection behavior is to only allow people with the maintain role to merge PRs. The permission differences are:

• Edit a repository's description
• Enable wikis and restrict wiki editors
• Enable project boards
• Configure pull request merges
• Configure a publishing source for GitHub Pages
• Push to protected branches
• Create tags that match a tag protection rule
• Create and edit repository social cards
• Enable GitHub Discussions in a repository

None of which I think are all that important to restrict (at least for this particular repo).

We could change it so that sync-team automatically includes people with the "write" role to be allowed to merge PRs. Or alternatively, to have a specific opt-in (like allow-write-role=true). However, I am not feeling inclined to hack on sync-team right now, and it would change permissions for backtrace-rs (adds alexcrichton), flate2 (adds brson), and impl-trait-utils (adds wg-async), which I don't want to fuss with.

Another option is to drop the branch protection, but at least I feel more comfortable with it enabled since it requires everything to go through a PR which is more publicly visible.

Also, just in general, it looks like we have been giving out maintain to almost all repos anyways.

[Mark-Simulacrum]

Agreed that this makes sense, and is a reasonable first point even if we do adjust branch protections in the future.