chore(python): update sdk.yaml with non-default release levels (#4850)

The language-default release level is often different to the current
Python library release level, as determined by the existing
.repo-metadata.json files. The sdk.yaml file is updated to reflect the
current state of play in google-cloud-python, so that after migration
we still generate the same .repo-metadata.json files.

Also adds google/maps/geocode/v4 to the allow-list, as that's now in
google-cloud-python.

Fixes #4847.

Author: jskeet

internal/librarian/python/generate_test.go

@@ -1120,7 +1120,7 @@ func TestCreateRepoMetadata(t *testing.T) {
 				NamePretty:           "Google Meet",
 				ProductDocumentation: "https://developers.google.com/meet/api/guides/overview",
 				IssueTracker:         "https://issuetracker.google.com/issues/new?component=1216362&template=1766418",
-				ReleaseLevel:         "stable",
+				ReleaseLevel:         "preview",
 				Language:             config.LanguagePython,
 				Repo:                 "googleapis/google-cloud-python",
 				DistributionName:     "google-apps-meet",