Fix bug of DisableGravatar default value (#22296)

#18058 made a mistake. The disableGravatar's default value depends on
`OfflineMode`. If it's `true`, then `disableGravatar` is true, otherwise
it's `false`. But not opposite.

Co-authored-by: KN4CK3R <admin@oldschoolhack.me>

modules/setting/picture.go

@@ -68,7 +68,7 @@ func newPictureService() {
 }
 
 func GetDefaultDisableGravatar() bool {
-	return !OfflineMode
+	return OfflineMode
 }
 
 func GetDefaultEnableFederatedAvatar(disableGravatar bool) bool {