refactor: extract fallback tag from versions.txt instead of workflow yaml

Author: meltsufin

generation/run_generator_docker.sh

@@ -46,8 +46,8 @@ if [[ "$CURRENT_BRANCH" =~ ^release-please-- ]]; then
   echo "Detected release PR branch: $CURRENT_BRANCH"
   if ! docker pull "${IMAGE_NAME}:${IMAGE_TAG}"; then
     echo "Image not found for version ${IMAGE_TAG}. Falling back to previous version from ${TARGET_BRANCH}."
-    # Extract tag from target branch's workflow file
-    PREVIOUS_TAG=$(git show "${TARGET_BRANCH}":.github/workflows/hermetic_library_generation.yaml | grep -m 1 "^[[:space:]]*image_tag:" | cut -d ':' -f 2- | cut -d '#' -f 1 | xargs || true)
+    # Extract tag from target branch's versions.txt
+    PREVIOUS_TAG=$(git show "${TARGET_BRANCH}":versions.txt | grep "^gapic-generator-java:" | cut -d ':' -f 2 || true)
     if [ -n "$PREVIOUS_TAG" ]; then
       echo "Using previous image version: $PREVIOUS_TAG"
       IMAGE_TAG="$PREVIOUS_TAG"