chore(ci): change to use the starting point of the previous branch fo…

…r old plugin compatibility test (#13536)

When a new minor version is just bumped, the tag x.x.0 doesn't exist
yet. So we change it to use the starting point of the previous branch
instead of the tag x.x.0 of the previous branch.

.github/workflows/build_and_test.yml

@@ -51,11 +51,18 @@ jobs:
         KONG_VERSION=$(bash scripts/grep-kong-version.sh)
         major=$(echo "$KONG_VERSION" | cut -d. -f1)
         minor=$(echo "$KONG_VERSION" | cut -d. -f2)
-        # if the minor version isn't 0, use the first release of the previous minor version;
+        # if the minor version isn't 0, use the first release or starting point of the previous minor branch;
         # otherwise just leave it empty, so later the default branch or commit will be used.
         if [ "$minor" -ne 0 ]; then
           minor=$((minor - 1))
-          echo "ref=$major.$minor.0" >> $GITHUB_OUTPUT
+          git fetch origin master -t
+          if [ $(git tag -l "$major.$minor.0") ]; then
+              echo "ref=$major.$minor.0" >> $GITHUB_OUTPUT
+          else
+              git fetch origin release/$major.$minor.x
+              COMMIT_HASH=$(git merge-base origin/master origin/release/$major.$minor.x)
+              echo "ref=$COMMIT_HASH" >> $GITHUB_OUTPUT
+          fi
         else
           echo "ref=" >> $GITHUB_OUTPUT
         fi