FindRuby: Restore compatibility variable RUBY_INCLUDE_PATH

Since commit e672db628b (FindRuby: Rename variables to match case of
module name, 2020-03-11, v3.18.0-rc1~546^2), the result variables named
with the old `RUBY_` prefix are provided by compatibility code that maps
from the new `Ruby_` prefix variables.  There is no `Ruby_INCLUDE_PATH`
variable, so do not try to map it to `RUBY_INCLUDE_PATH`.  The latter is
provided by dedicated compatibility code left from before that
transition.

Fixes: #21064

Modules/FindRuby.cmake

@@ -50,7 +50,7 @@ This module will set the following variables in your project:
 
 Also:
 
-``Ruby_INCLUDE_PATH``
+``RUBY_INCLUDE_PATH``
   same as Ruby_INCLUDE_DIRS, only provided for compatibility reasons, don't use it
 
 Hints
@@ -493,7 +493,6 @@ foreach(Camel
     Ruby_VERSION_MAJOR
     Ruby_VERSION_MINOR
     Ruby_VERSION_PATCH
-    Ruby_INCLUDE_PATH
 
     Ruby_ARCH_DIR
     Ruby_ARCH