Align go directive to 1.26 line for latest Red Hat Go availability

The x/net bump in the previous commit raised the go.mod "go" directive
to 1.25.0 as an incidental side effect of satisfying x/net v0.55.0's
own module requirements. Aligning explicitly to the 1.26 line instead,
since Red Hat now has newer Go 1.26.x builds available for this
platform than what's currently tagged into this stream's Brew
buildroot override (golang-1.26.4-1.el9_8, dbenoit-built, already
shipped as RHSA-2026:29981 for other products) -- tagging it into
this stream's override would also satisfy the Go >= 1.26.3 fix
threshold for CVE-2026-33811 and CVE-2026-39820.

Ref: HELM-816, HELM-817

Author: baijum

go.mod

@@ -1,6 +1,6 @@
 module helm.sh/helm/v3
 
-go 1.25.0
+go 1.26
 
 require (
 	github.com/AdaLogics/go-fuzz-headers v0.0.0-20230811130428-ced1acdcaa24