Use squash merge strategy for mergify (#7455)
Use squash merge strategy for mergify (#7455)

This matches how we usually merge other PRs: Merge strategy in general,
except for single commit PRs, where we use squash. Dependabot PRs are
single commit, so we should use squash for them too.
activeadmin/activeadmin,activeadmin/activeadmin,activeadmin/activeadmin