.github: add Go 1.20

Go 1.20 was released in February - 4 months ago:

https://go.dev/doc/devel/release

Change-Id: Id24b29f3f7009d34b3ec082adf623dfc41431876

.github/workflows/ci.yml

@@ -15,6 +15,7 @@ jobs:
           - "1.17.x" # Golang upstream stable
           - "1.18.x" # Golang upstream stable
           - "1.19.x" # Golang upstream stable
+          - "1.20.x" # Golang upstream stable
       # Don't cancel everything when one Go version fails
       fail-fast: false
     runs-on: ubuntu-latest