Revert: "TEMP be sure we run against rails 6" (#3521)

Reverted 1430e8f - "TEMP be sure we run
against rails 6"
Seems like this shouldn't have been left in the code.
The main readme says `An open source content management system for Rails
5.1+`
So far as I can tell, refinery should support at least rails 5.2 and
rails 6.X

If this is not a correct understanding, I would urge the readme to be
updated appropriately.

Author: matthewhively

core/refinerycms-core.gemspec

@@ -3,7 +3,7 @@
 require File.expand_path('../core/lib/refinery/version', __dir__)
 
 version = Refinery::Version.to_s
-rails_version = ['>= 6.0.0', '< 7']
+rails_version = ['>= 5.2.0', '< 7']
 
 Gem::Specification.new do |s|
   s.platform          = Gem::Platform::RUBY