Release on only one Go version (#6637)

Co-authored-by: Aaron Gable <aaron@letsencrypt.org>

Author: jsha

.github/workflows/release.yml

@@ -1,7 +1,8 @@
 # Build the Boulder Debian package on every PR, push to main, and tag push. On
 # tag pushes, additionally create a GitHub release and with the resulting Debian
 # package.
-# Keep in sync with try-release.yml
+# Keep in sync with try-release.yml, with the exception that try-release.yml
+# can have multiple entries in its matrix but this should only have one.
 name: Build release
 on:
   push:
@@ -15,7 +16,6 @@ jobs:
       matrix:
         GO_VERSION:
           - "1.19.5"
-          - "1.20"
     runs-on: ubuntu-20.04
     permissions:
       contents: write