chore: Move changelog up, fix 27 upgrade headings

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

Author: ChristophWurst

admin_manual/release_notes/index.rst

@@ -4,6 +4,9 @@ Release notes
 
 .. _critical-changes:
 
+Critical changes
+----------------
+
 Once you've installed and configured your server, you will want to keep it up to date with the latest Nextcloud features.
 
 These sub pages will cover the most important changes in Nextcloud, as well as some guides on how to upgrade existing installations.
@@ -12,3 +15,8 @@ These sub pages will cover the most important changes in Nextcloud, as well as s
    :maxdepth: 1
 
    upgrade_to_27.rst
+
+Changelog
+---------
+
+See `the official changelog <https://nextcloud.com/changelog/>`_ for a complete list of changes.

admin_manual/release_notes/upgrade_to_27.rst

@@ -3,16 +3,11 @@ Upgrade to Nextcloud 27
 =======================
 
 System requirements
-^^^^^^^^^^^^^^^^^^^
+-------------------
 
 * PHP8.2 is recommended over PHP8.1.
 
 Exposed system address book
-^^^^^^^^^^^^^^^^^^^^^^^^^^^
+---------------------------
 
-Nextcloud 27 exposes the :ref:`system address book<system-address-book>`. Restrict the enumeration settings if your users should not see other users.
-
-Changelog
----------
-
-See `the official changelog <https://nextcloud.com/changelog/>`_ for a complete list of changes.
+Nextcloud 27 exposes the :ref:`system address book<system-address-book>`. Restrict the enumeration settings if your users should not see other users.