Use Constants.MAX_CHANNELNAME_LENGTH where a fixed number like 22 or 64 is used for max length of channel names. (#7041)

webapp/i18n/en.json

@@ -1982,7 +1982,7 @@
   "rename_channel.displayNameHolder": "Enter display name",
   "rename_channel.handleHolder": "lowercase alphanumeric characters",
   "rename_channel.lowercase": "Must be lowercase alphanumeric characters",
-  "rename_channel.maxLength": "This field must be less than 22 characters",
+  "rename_channel.maxLength": "This field must be less than {maxLength, number} characters",
   "rename_channel.required": "This field is required",
   "rename_channel.save": "Save",
   "rename_channel.title": "Rename Channel",