fix(migration): move serverid into the bullet list

Keep consistent structure: serverid belongs in the checklist alongside
other keys to verify, not as a separate paragraph after secret/instanceid.

Signed-off-by: skjnldsv <skjnldsv@protonmail.com>

Author: skjnldsv

admin_manual/maintenance/migrating.rst

@@ -106,16 +106,15 @@ the new location. It is also assumed that the authentication method
       * ``logfile`` — update if the log path differs on the new server.
       * ``tempdirectory`` — if set to a custom path, make sure that path exists and
         is writable on the new server.
+      * ``serverid`` — if set, keep the same value. It identifies the server in
+        multi-PHP-server setups and must not change. If you override it per server via
+        the ``NC_serverid`` environment variable, configure the same override on the new
+        server.
 
       The ``secret`` and ``instanceid`` values are generated once at install time and
       tied to all encrypted data and user sessions. **Do not change or regenerate them.**
       They must be copied verbatim from the original ``config.php``.
 
-      If ``serverid`` is set, keep the same value. It identifies the server in
-      multi-PHP-server setups and must not change. If you override it per server via
-      the ``NC_serverid`` environment variable, configure the same override on the new
-      server.
-
       Leaving stale values (especially database credentials or ``datadirectory``)
       will cause startup errors or data corruption.