(MAINT) Addressing wrong Rubocop TargetRubyVersion

Prior to this commit, during the Puppet 8 support work, TargetRubyVersion
was set to 2.7 and a cop rule was addressed according to it. However,
the correct ruby version is 2.6.

This commit aims to address the previously wrongly set version along with
an unnecesary change that breaks some compatibility with 2.6 systems.

Author: LukasAud

.rubocop.yml

@@ -7,7 +7,7 @@ require:
 AllCops:
   DisplayCopNames: true
   NewCops: enable
-  TargetRubyVersion: '2.7'
+  TargetRubyVersion: '2.6'
   ExtraDetails: true
   DisplayStyleGuide: true
   Include:

lib/puppet/type/concat_file.rb

@@ -190,14 +190,14 @@ def exists?
 
   def fragments
     # Collect fragments that target this resource by path, title or tag.
-    @fragments ||= catalog.resources.filter_map do |resource|
+    @fragments ||= catalog.resources.map { |resource|
       next unless resource.is_a?(Puppet::Type.type(:concat_fragment))
 
       if resource[:target] == self[:path] || resource[:target] == title ||
          (resource[:tag] && resource[:tag] == self[:tag])
         resource
       end
-    end
+    }.compact
   end
 
   def decompound(d)