fix(ci): skip existing-versions-check for gapic-generator-java-root (#13590)

Fixes #12840 by
correctly skipping the sdk-platform-java root POM from the duplicate
release versions check.

Author: jinseopkim0

generation/check_existing_release_versions.sh

@@ -44,7 +44,7 @@ function find_existing_version_pom() {
 return_code=0
 
 for pom_file in $(find . -maxdepth 3 -name pom.xml|sort --dictionary-order); do
-  if [[ "${pom_file}" == *java-samples* || "${pom_file}" == *showcase* || "${pom_file}" == *coverage-report* || "${pom_file}" == *gapic-generator-java-root* ]]; then
+  if [[ "${pom_file}" == *java-samples* || "${pom_file}" == *showcase* || "${pom_file}" == *coverage-report* || "${pom_file}" == *sdk-platform-java/pom.xml ]]; then
       continue
   fi
   find_existing_version_pom "${pom_file}"