[Infra] Release testing should grab latest tag on any branch (#13769)

scripts/release_testing_setup.sh

@@ -25,7 +25,10 @@ git fetch --tags --quiet origin main
 git checkout main
 
 # The chunk below is to determine the latest version by searching
-# Get the latest released tag Cocoapods-X.Y.Z for release and prerelease testing, beta version will be excluded.
+# Get the latest released tag Cocoapods-X.Y.Z for release and prerelease
+# testing, beta version will be excluded.
+# Note: If the nightly tag was not updated, check that the next release's tag
+# is on the main branch.
 test_version=$(git tag -l --sort=-version:refname --merged main 'CocoaPods-*[0-9]' | head -n 1)
 if [ -z "$test_version" ]; then
   echo "Latest tag could not be found. Exiting." >&2