Update Poetry to 2.1.1, including updating the lock file version. (#18251)

.ci/scripts/check_lockfile.py

@@ -11,12 +11,12 @@ with open("poetry.lock", "rb") as f:
 
 try:
     lock_version = lockfile["metadata"]["lock-version"]
-    assert lock_version == "2.0"
+    assert lock_version == "2.1"
 except Exception:
     print(
         """\
-    Lockfile is not version 2.0. You probably need to upgrade poetry on your local box
-    and re-run `poetry lock --no-update`. See the Poetry cheat sheet at
+    Lockfile is not version 2.1. You probably need to upgrade poetry on your local box
+    and re-run `poetry lock`. See the Poetry cheat sheet at
     https://element-hq.github.io/synapse/develop/development/dependencies.html
     """
     )