[RH8] Show warning message when site.xcatsslversion has a value during upgrade

[neo954]

The PR is to fix issue #6089

The modification include

##Show warning message when site.xcatsslversion has an value during upgrade

##Documentation updates

[neo954]

The unit test result.

# xcatconfig -u
__      ___   ___ _  _ ___ _  _  ___
\ \    / /_\ | _ \ \| |_ _| \| |/ __|                      _     +-+-+-+-+-+-+-+
 \ \/\/ / _ \|   / .` || || .` | (_ |                     oo\    |W|A|R|N|I|N|G|
  \_/\_/_/ \_\_|_\_|\_|___|_|\_|\___|                    (__)\   +-+-+-+-+-+-+-+
+--------------------------------------------------------------+ +-+-+-+-+-+-+-+
| The default value of `site.xcatsslversion' was updated.      |:|W|A|R|N|I|N|G|
| Run `chdef -t site xcatsslversion=' to update your system to |:+-+-+-+-+-+-+-+
| the new default value. See `man site' for more details.      |:+-+-+-+-+-+-+-+
+--------------------------------------------------------------+:|W|A|R|N|I|N|G|
 ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::+-+-+-+-+-+-+-+

[immarvin]

i am ok with the change