Update CI runner to ubuntu-20.04 (#10835)

ubuntu-18.04 is going to be deprecated in Dec. 2022.
diff --git a/.github/workflows/validate_and_publish_release.yml b/.github/workflows/validate_and_publish_release.yml
index f328db6..cef78b6 100644
--- a/.github/workflows/validate_and_publish_release.yml
+++ b/.github/workflows/validate_and_publish_release.yml
@@ -19,7 +19,7 @@
   validate_packages:
     name: "Validate packages"
     # TODO(jennik): Look into testing windows and macos builds.
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - name: Download packages
         id: download_packages
@@ -73,7 +73,7 @@
   publish_release:
     name: "Publish release"
     needs: validate_packages
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - name: Publish Release
         id: publish_release