.github/workflows/ci.yml: update Go versions

Because we aim to build and test against only supported versions
of Go, we upgrade our GitHub Actions CI workflows to test against
Go versions 1.26 and 1.25.

Author: chrisd8088

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ jobs:
     name: Default build
     strategy:
       matrix:
-        go: ['1.20.x', '1.21.x']
+        go: ['1.25.x', '1.26.x']
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v7