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

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

server/channels/app/import_functions_test.go

@@ -1512,7 +1512,6 @@ func TestImportImportUser(t *testing.T) {
 
 	// see https://mattermost.atlassian.net/browse/MM-56986
 	// Test importing deleted guest with a valid team & valid channel name in apply mode.
-	// mlog.Debug("TESTING GUEST")
 	// username = model.NewId()
 	// deleteAt = model.GetMillis()
 	// deletedGuestData := &imports.UserImportData{
@@ -1537,7 +1536,6 @@ func TestImportImportUser(t *testing.T) {
 
 	// user, appErr = th.App.GetUserByUsername(*deletedGuestData.Username)
 	// require.Nil(t, appErr, "Failed to get user from database.")
-	// mlog.Debug(user.Roles)
 
 	// teamMember, appErr = th.App.GetTeamMember(th.Context, team.Id, user.Id)
 	// require.Nil(t, appErr, "Failed to get the team member")