We disabled Dependabot in commit a8d6b9b33 because it would
automatically create pull requests on every fork.
But since November 2022 Dependabot no longer behaves that way [1], so
now we can safely enable it without spamming every repository in our
community.
[1] https://github.blog/changelog/2022-11-07-dependabot-pull-requests-off-by-default-for-forks/