ldap: make uid the default account name

I fail to understand how mail became the uidAttribute way back when LDAP
support was introduced, but it was unintentional and clearly a mistake.

The uid attribute is the standard system login name per RFC4519 2.39 and
what we default to going forward.

docs/release-notes.rst

@@ -22,6 +22,11 @@ NixOS 26.05
   established by `agenix`_/`sops-nix`_ that instead rely on encryption. This
   option prevents files from leaking in to the Nix store.
   See :option:`mailserver.loginAccounts.<name>.passwordFile`.
+- The default login username for LDAP users has changed from the ``mail`` to
+  the ``uid`` attribute. This allows users to login with their account name
+  rather than their email address, which is more convenient and consistent
+  with typical LDAP practices. The exact attribute can be customized through
+  :option:`mailserver.ldap.attributes.username`.
 - The following integrations are deprecated and will be removed before the next
   release: