MM-22051: Remove ToJson methods from network writes (#17999)

We replace the double conversion of
[]byte to string, with a direct write
to http.ResponseWriter.

https://mattermost.atlassian.net/browse/MM-22051

Tried using gofmt -r, but it only accepts Go
expressions. So had to resort to an ugly sed replace

sed -E -i 's/w.Write\(\[\]byte\((.*).ToJson\(\)\)\)/if err := json.NewEncoder\(w\).Encode\(\1\); err != nil { mlog.Warn\("Error while writing response", mlog.Err\(err\)\)}/g' *.go

```release-note
NONE
```

api4/upload.go

@@ -4,6 +4,7 @@
 package api4
 
 import (
+	"encoding/json"
 	"errors"
 	"io"
 	"mime/multipart"
@@ -11,6 +12,7 @@ import (
 
 	"github.com/mattermost/mattermost-server/v6/audit"
 	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 func (api *API) InitUpload() {
@@ -66,7 +68,9 @@ func createUpload(c *Context, w http.ResponseWriter, r *http.Request) {
 
 	auditRec.Success()
 	w.WriteHeader(http.StatusCreated)
-	w.Write([]byte(us.ToJson()))
+	if err := json.NewEncoder(w).Encode(us); err != nil {
+		mlog.Warn("Error while writing response", mlog.Err(err))
+	}
 }
 
 func getUpload(c *Context, w http.ResponseWriter, r *http.Request) {
@@ -86,7 +90,9 @@ func getUpload(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	w.Write([]byte(us.ToJson()))
+	if err := json.NewEncoder(w).Encode(us); err != nil {
+		mlog.Warn("Error while writing response", mlog.Err(err))
+	}
 }
 
 func uploadData(c *Context, w http.ResponseWriter, r *http.Request) {
@@ -136,7 +142,9 @@ func uploadData(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	w.Write([]byte(info.ToJson()))
+	if err := json.NewEncoder(w).Encode(info); err != nil {
+		mlog.Warn("Error while writing response", mlog.Err(err))
+	}
 }
 
 func doUploadData(c *Context, us *model.UploadSession, r *http.Request) (*model.FileInfo, *model.AppError) {