build: Test against Go 1.25.

This updates CI to test against Go 1.24 and Go 1.25 and removes the
tests for Go 1.19 and Go 1.20 accordingly.

Author: davecgh

.github/workflows/go.yml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go: ["1.19", "1.20"]
+        go: ["1.24", "1.25"]
     steps:
       - name: Check out source
         uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0