Update supported go versions to 1.25+1.26

Follow the release of go 1.26.0 today, go 1.24 is no longer supported by
upstream.
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index fda1b9d..058793c 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -173,8 +173,8 @@
     strategy:
       matrix:
         go:
-          - '1.24'
           - '1.25'
+          - '1.26'
       fail-fast: false
     steps:
       - uses: actions/checkout@v6
@@ -218,7 +218,7 @@
         run: make -C test/go precross
 
       - name: Upload go precross artifacts
-        if: matrix.go == '1.25'
+        if: matrix.go == '1.26'
         uses: actions/upload-artifact@v6
         with:
           name: go-precross