Remove SHA1 for support for ssh rsa signing (#31857)

https://github.com/go-fed/httpsig seems to be unmaintained. 

Switch to github.com/42wim/httpsig which has removed deprecated crypto
and default sha256 signing for ssh rsa.

No impact for those that use ed25519 ssh certificates.

This is a breaking change for:
- gitea.com/gitea/tea (go-sdk) - I'll be sending a PR there too
- activitypub using deprecated crypto (is this actually used?)

modules/setting/federation.go

@@ -6,7 +6,7 @@ package setting
 import (
 	"code.gitea.io/gitea/modules/log"
 
-	"github.com/go-fed/httpsig"
+	"github.com/42wim/httpsig"
 )
 
 // Federation settings