fix: update golang version in buildpack-integration-workflow-validation (#178)

Author: akerekes

.github/workflows/buildpack-integration-workflow-validation.yml

@@ -11,11 +11,11 @@ jobs:
                 builder-url: ['gcr.io/gae-runtimes/buildpacks/go/builder:latest', '']
         uses: ./.github/workflows/buildpack-integration-test.yml
         with:
-            builder-runtime: 'go113'
-            builder-runtime-version: '1.13'
+            builder-runtime: 'go120'
+            builder-runtime-version: '1.20'
             http-builder-source: 'testdata'
             http-builder-target: 'HTTP'
             cloudevent-builder-source: ''
             cloudevent-builder-target: ''
             builder-url: ${{ matrix.builder-url }}
-            conformance-client-version: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
+            conformance-client-version: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}