[Infra] Get testing tag from latest tag merged to main (#13061)

Author: ncooke3

scripts/release_testing_setup.sh

@@ -32,7 +32,7 @@ set -x
 cd  "${local_sdk_repo_dir}"
 # The chunk below is to determine the latest version by searching
 # Get the latest released tag Cocoapods-X.Y.Z for release and prerelease testing, beta version will be excluded.
-test_version=$(git tag -l --sort=-version:refname CocoaPods-*[0-9] | head -n 1)
+test_version=$(git tag -l --sort=-version:refname --merged main CocoaPods-*[0-9] | head -n 1)
 
 git config --global user.email "[email protected]"
 git config --global user.name "google-oss-bot"