conf.py: remove 2023 from copyright date range

Problem: The copyright dates on flux-docs specifies a range of years
(2014-2023), but this copyright is still active.

Remove 2023 from the copyright and just leave the starting year.

Author: cmoussa1

conf.py

@@ -31,7 +31,7 @@
 # -- Project information -----------------------------------------------------
 
 project = "Flux"
-copyright = """2014-2023 Lawrence Livermore National Security, LLC and Flux developers.
+copyright = """2014 Lawrence Livermore National Security, LLC and Flux developers.
 
 SPDX-License-Identifier: LGPL-3.0"""
 author = "This page is maintained by the Flux Framework community."