MM-52638- Last admin cannot be demoted (#24087)

* don't allow last sysadmin to change roles

* cleanup, add comment

* only allow admin downgrade if more than one admin

* remove unused variable

* i18n-extract, unit test fixes

* Update user.go

* remove blank line

* update tests check all return values

* revert channel_store.go

---------

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

server/i18n/en.json

@@ -6846,6 +6846,10 @@
     "id": "app.user.store_is_empty.app_error",
     "translation": "Failed to check if user store is empty."
   },
+  {
+    "id": "app.user.update.countAdmins.app_error",
+    "translation": "Error determining number of System Admin accounts."
+  },
   {
     "id": "app.user.update.find.app_error",
     "translation": "Unable to find the existing account to update."
@@ -6854,6 +6858,10 @@
     "id": "app.user.update.finding.app_error",
     "translation": "We encountered an error finding the account."
   },
+  {
+    "id": "app.user.update.lastAdmin.app_error",
+    "translation": "Cannot demote last System Admin."
+  },
   {
     "id": "app.user.update_active.user_limit.exceeded",
     "translation": "Can't activate user. Server exceeds safe user limit. Contact your administrator with: ERROR_SAFETY_LIMITS_EXCEEDED."