It seems that there's no way to just enable auto-merge without
specifying a merge strategy (presumably because some projects allow
several GitHub merge strategies), so I guess we'll have to live with
the strategy being duplicated between here and the project settings.
To merge a Dependabot PR, I have to enable auto-merge (two clicks,
including one to confim) and then review and approve it. Since our
branch protections require the PR to be approved, it seems that that
should be enough. This patch adds a GitHub action that calls runs the
GitHub CLI to do that. It is based on
https://dev.to/slashgear_/how-to-automatically-merge-dependabot-pull-requests-with-github-actions--30pe
