Cleanup building header files for different OS

'make distclean' removes header files built from the .in files, which
means that they are not available for different OS builds.

The same is true for CMakeLists.txt.

Remove the need for the .in files and update for libcoap revisions
by using a new script scripts/fix_version.sh created from
scripts/fix_version.sh.in.

CI has been updated to check revisions have been updated in the
appropriate places.

Do not remove libcoap-3.sym and libcoap-3.map when doing 'make distclean'
as they are git tracked files.

CMakeLists.txt

@@ -1,9 +1,5 @@
 # CMakeLists.txt for libcoap
 #
-# [CMakeLists.txt is re-built from CMakeLists.txt.in by the libcoap
-#  ./configure command, so make any permanent changes to libcoap's
-#  CMakeLists.txt.in for a libcoap Pull Request]
-#
 # Copyright (C) 2020 Carlos Gomes Martinho <carlos.gomes_martinho@siemens.com>
 # Copyright (C) 2020-2024 Jon Shallow <supjps-libcoap@jpshallow.com>
 #