Add missing //nolint:depguard (go-gitea#37162)

When running `golangci-lint` without `GOEXPERIMENT=jsonv2`, a lint error
`import 'encoding/json' is not allowed` is seen.

All other files in the module that import `encodings/json` have
`//nolint` already, so add it.

---
This PR was written with the help of Claude Opus 4.6

Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>

Author: 2 people

modules/json/jsonlegacy.go

@@ -6,7 +6,7 @@
 package json
 
 import (
-	"encoding/json"
+	"encoding/json" //nolint:depguard // this package wraps it
 	"io"
 )