Use Ubuntu 20.04 in trusted publishing workflow

The release workflow ran on ubuntu-22.04 with Ruby 3.3.3 and failed, we
haven't tested that combo yet.

Hopefully this works as we do test on ubuntu-20.04 and Ruby 3.3.

.github/workflows/release.yml

@@ -8,7 +8,7 @@ on:
 jobs:
   release:
     if: github.repository == 'sporkmonger/addressable'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     permissions:
       id-token: write # for trusted publishing
     steps: