Upgrade our C++ release tests to large runners.

PiperOrigin-RevId: 647405381

Author: mkruskal-google

.github/workflows/test_cpp.yml

@@ -77,7 +77,7 @@ jobs:
       matrix:
         arch: [x86_64, aarch64]
     name: Linux Release ${{ matrix.arch}}
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20-large
     steps:
       - name: Checkout pending changes
         uses: protocolbuffers/protobuf-ci/checkout@v2