[fluentd] update main repo to 1.19.0

Author: ThomsonTan

.github/workflows/fluentd.yml

@@ -29,7 +29,7 @@ jobs:
         uses: actions/checkout@v3
         with:
           repository: "open-telemetry/opentelemetry-cpp"
-          ref: "v1.15.0"
+          ref: "v1.19.0"
           path: "opentelemetry-cpp"
           submodules: "recursive"
       - name: setup dependencies