Restore retire workers API

[fjetter]

#8935 accidentally changed the public API for retire workers. I thought this was used only internally but it is used by client.retire_workers as well (and of course whenever the request handler is being used directly)

This should restore the API without reverting the changes to the logged event that was the primary intention of #8935

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

    25 files  ±0      25 suites  ±0   10h 21m 30s ⏱️ + 5m 9s
 4 130 tests ±0   4 017 ✅ +3    110 💤 ±0   3 ❌  - 2 
47 692 runs   - 1  45 555 ✅  - 9  2 122 💤 ±0  15 ❌ +9 

For more details on these failures, see this check.

Results for commit e16728f. ± Comparison against base commit d7eff77.

[jrbourbeau]

Thanks @fjetter

Looks like we need to update distributed/tests/test_active_memory_manager.py::test_RetireWorker_with_actor_proxy too

diff --git a/distributed/tests/test_active_memory_manager.py b/distributed/tests/test_active_memory_manager.py
index bcc0d48d..1bc9855b 100644
--- a/distributed/tests/test_active_memory_manager.py
+++ b/distributed/tests/test_active_memory_manager.py
@@ -1250,7 +1250,7 @@ async def test_RetireWorker_with_actor_proxy(c, s, a, b):
     assert "y" in b.data

     out = await c.retire_workers([b.address])
-    assert out == (b.address,)
+    assert out.keys() == {b.address}
     assert "x" in a.state.actors
     assert "y" in a.data