Update CI matrix to Go 1.25 only

Go 1.23 and 1.24 are incompatible with go.mod requiring 1.25.8,
and stdlib CVE fixes are only available in 1.25+.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mason-sharp

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
         runs-on: ubuntu-latest
         strategy:
             matrix:
-                go-version: ['1.23', '1.24', '1.25']
+                go-version: ['1.25']
 
         steps:
             - name: Checkout code