ref(cells): remove support for region_name in ByCellName resolver

[lynnagara]

all cell rpc methods that use the ByCellName resolver have been migrated to cell_name. We can drop support for region_name now.

[github-actions]

Backend Test Failures

Failures on 3cb8478 in this run:

tests/sentry/hybridcloud/test_cell.py::CellResolutionTest::test_by_cell_name — log

tests/sentry/hybridcloud/test_cell.py:35: in test_by_cell_name
    assert resolver.resolve({"region_name": self.target_cell.name}) == self.target_cell
src/sentry/hybridcloud/rpc/resolvers.py:47: in resolve
    cell_name = arguments[self.parameter_name]
E   KeyError: 'cell_name'