chore: update go version to 1.25.8 (#2514)

Applies various security patches and bug fixes:
https://go.dev/doc/devel/release#go1.25.minor

Also:

* Update CONTRIBUTING to indicate to use -s flag when signing commits

Signed-off-by: eadred <eadred77@googlemail.com>

CONTRIBUTING.md

@@ -19,7 +19,7 @@ $ git checkout -b your-shiny-new-feature origin/main
 
 ...
 
-$ git commit -m 'feat: do whatever for whatever purpose
+$ git commit -s -m 'feat: do whatever for whatever purpose
 
 This adds ... by:
 
@@ -30,6 +30,9 @@ This adds ... by:
 Resolves #ISSUE_NUMBER
 '
 
+Remember to include a `Signed-off-by: Author Name <authoremail@example.com>`
+line in the commit message, either manually or using the `-s` flag.
+
 $ hub fork
 $ git push YOUR_GITHUB_USER your-shiny-new-feature
 $ hub pull-request

go.mod

@@ -1,6 +1,6 @@
 module github.com/helmfile/helmfile
 
-go 1.25.4
+go 1.25.8
 
 require (
 	dario.cat/mergo v1.0.2