Move to Bookstack 23.12.3

See https://github.com/BookStackApp/BookStack/releases/tag/v23.12 for
the list of improvements for 23.12.  The .1 and .2 releases are smaller
bugfixes, and .3 brings a security fix related to PDF generation.

This also brings in two SAML-related improvements
(BookStackApp/BookStack#4713 and BookStackApp/BookStack#4706).  Thanks
again ssddanbrown for them!  And thanks also to the LinuxServer folks
for the packaging.

Dockerfile

@@ -1,7 +1,7 @@
 # vim: sw=2 ts=2 et
 
 # Start with the linuxserver bookstack container
-FROM lscr.io/linuxserver/bookstack:amd64-23.10.4
+FROM lscr.io/linuxserver/bookstack:amd64-23.12.3
 
 # Copy static files into the container:
 #  * The periodic script (runs daily) to do a Certbot renewal