Update copyright year

(cherry picked from commit a83ad1c7)

common/xfpm-common.c

@@ -110,7 +110,7 @@ xfpm_about (gpointer data)
   };
 
   gtk_show_about_dialog (NULL,
-       "copyright", "Copyright \302\251 2008-2011 Ali Abdallah\nCopyright \302\251 2011-2012 Nick Schermer\nCopyright \302\251 2013-2015 Eric Koegel, Harald Judt, Simon Steinbeiß",
+       "copyright", "Copyright \302\251 2008-2011 Ali Abdallah\nCopyright \302\251 2011-2012 Nick Schermer\nCopyright \302\251 2013-2015 Eric Koegel, Harald Judt, Simon Steinbeiß\nCopyright \302\251 2016-2023 The Xfce development team",
        "destroy-with-parent", TRUE,
        "authors", authors,
        "artists", artists,

configure.ac.in

@@ -6,6 +6,7 @@ m4_define([xfpm_version_build], [@REVISION@])
 m4_define([xfpm_version_tag], [git])
 m4_define([xfpm_version], [xfpm_version_major().xfpm_version_minor().xfpm_version_micro()ifelse(xfpm_version_tag(), [git], [xfpm_version_tag()-xfpm_version_build()], [xfpm_version_tag()])])
 
+AC_COPYRIGHT([Copyright (c) 2008-2023 The Xfce development team. All rights reserved.])
 AC_INIT([xfce4-power-manager],[xfpm_version],[http://bugzilla.xfce.org/])
 AC_PREREQ([2.60])
 AC_CONFIG_MACRO_DIRS([m4])