MM 56929 Dont allow guests to be set via channel API (#26385)

* don't allow guest to be set only on channels.

* fix bad merge, add jira ticket link

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/i18n/en.json

@@ -451,6 +451,10 @@
     "id": "api.channel.update_channel_member_roles.changing_guest_role.app_error",
     "translation": "Invalid channel member update: You can't add or remove the guest role manually."
   },
+  {
+    "id": "api.channel.update_channel_member_roles.guest.app_error",
+    "translation": "Invalid channel member update: A guest cannot be made team member or team admin, please promote as a user first."
+  },
   {
     "id": "api.channel.update_channel_member_roles.guest_and_user.app_error",
     "translation": "Invalid channel member update: A user must be a guest or a user but not both."
@@ -459,6 +463,10 @@
     "id": "api.channel.update_channel_member_roles.scheme_role.app_error",
     "translation": "The provided role is managed by a Scheme and therefore cannot be applied directly to a Channel Member."
   },
+  {
+    "id": "api.channel.update_channel_member_roles.user_and_guest.app_error",
+    "translation": "Invalid channel member update: A guest cannot be set for a single channel, a System Admin must promote or demote users to/from guests."
+  },
   {
     "id": "api.channel.update_channel_privacy.default_channel_error",
     "translation": "The default channel cannot be made private."