Switch to Firedrake's `_Facets.unique_markers` from removed `.markers`

[inducer]

https://github.com/firedrakeproject/firedrake/commit/9125a65c0cb5bb671c62c33f05a0d42b983e06ed

Representative CI failure: https://github.com/inducer/meshmode/actions/runs/4395544902/jobs/7697410990