ci: retarget buildpack integration test to main instead of release version

Author: HKWinterhalter

.github/workflows/buildpack-integration-test.yml

@@ -15,7 +15,7 @@ permissions: read-all
 
 jobs:
   ruby30-buildpack-test:
-    uses: GoogleCloudPlatform/functions-framework-conformance/.github/workflows/buildpack-integration-test.yml@v1.8.5
+    uses: GoogleCloudPlatform/functions-framework-conformance/.github/workflows/buildpack-integration-test.yml@main
     with:
       http-builder-source: 'test/conformance'
       http-builder-target: 'http_func'