Update Ubuntu instance

Author: josevalim

.github/workflows/release.yml

@@ -39,7 +39,7 @@ jobs:
           - otp: 25
             otp_version: 25.0
             build_docs: build_docs
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
         with: