Add GAMS version check

GAMS distribution should be 24.0 or alter to support e.g. double dollar commands.

Backbone.gms

@@ -72,6 +72,9 @@ References
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 $offtext
 
+* Check current GAMS version
+$ife %system.gamsversion%<240 $abort GAMS distribution 24.0 or later required!
+
 * Set default debugging level
 $if not set debug $setglobal debug 0