ci: standardize release branch pattern to release-X.Y.x

Update all CI workflows and actions to use the standardized release branch
naming convention 'release-X.Y.x' (e.g., release-0.3.x) instead of matching
multiple numeric patterns.

Changes:
- Update workflow triggers to match 'release-[0-9]+.[0-9]+.x' pattern
- Update bash regex in setup-test-environment to match new format
- Simplifies branch pattern from multiple variants to single standard

.github/actions/setup-test-environment/action.yml

@@ -47,7 +47,7 @@ runs:
           # Check if PR is targeting a release branch
           TARGET_BRANCH="${{ github.base_ref }}"
 
-          if [[ "$TARGET_BRANCH" =~ ^release-([0-9]+\.){1,3}[0-9]+$ ]]; then
+          if [[ "$TARGET_BRANCH" =~ ^release-[0-9]+\.[0-9]+\.x$ ]]; then
             echo "PR targets release branch: $TARGET_BRANCH"
             echo "Checking if matching branch exists in llama-stack-client-python..."