fix(core): remove duplicate MySQL user prefix

PR #53212 implemented a hardcoded database username, instead of deriving it from the Nextcloud admin username, to be able to setup Nextcloud without admin user. This has the additional benefit that knowing one of them does not allow to derive the other.

However, it used `oc_admin`, while the MySQL database setup adds `oc_` again in the dedicated `createSpecificUser()` function, resulting in `oc_oc_admin`. In case of PostgreSQL, this was done in `setupDatabase()`, replaced with the hardcoded username.

Signed-off-by: MichaIng <micha@dietpi.com>

Author: MichaIng

lib/private/Setup/MySQL.php

@@ -142,8 +142,8 @@ private function createSpecificUser(string $username, IDBConnection $connection)
 
 			//we don't have a dbuser specified in config
 			if ($this->dbUser !== $oldUser) {
-				//add prefix to the admin username to prevent collisions
-				$adminUser = substr('oc_' . $username, 0, 16);
+				//cap to 16 characters
+				$adminUser = substr($username, 0, 16);
 
 				$i = 1;
 				while (true) {
@@ -169,7 +169,7 @@ private function createSpecificUser(string $username, IDBConnection $connection)
 					} else {
 						//repeat with different username
 						$length = strlen((string)$i);
-						$adminUser = substr('oc_' . $username, 0, 16 - $length) . $i;
+						$adminUser = substr($username, 0, 16 - $length) . $i;
 						$i++;
 					}
 				}