chore: update Go version to 1.26.1

- Update GOLANG_VERSION in .github/env from 1.25.7 to 1.26.1
- Add comment in .github/env to keep version aligned with go.mod
- Update go directive in go.mod to 1.26.1

Author: l-qing

.github/env

@@ -1,2 +1,3 @@
-GOLANG_VERSION=1.25.7
+# Keep this version aligned with the `go` directive in go.mod.
+GOLANG_VERSION=1.26.1
 GOLANGCI_LINT_VERSION=v1.64

go.mod

@@ -1,6 +1,6 @@
 module helm.sh/helm/v3
 
-go 1.25.7
+go 1.26.1
 
 require (
 	github.com/AdaLogics/go-fuzz-headers v0.0.0-20230811130428-ced1acdcaa24