Suppress new Style/RedundantRegexpArgument offenses

This PR suppresses the following new `Style/RedundantRegexpArgument` offenses:

```console
$ bundle exec rake
(snip)

Offenses:

lib/rubocop/cop/performance/redundant_merge.rb:112:30: C: [Correctable] Style/RedundantRegexpArgument: Use string "\n" as argument instead of regexp /\n/.
            new_source.gsub!(/\n/, padding)
                             ^^^^
lib/rubocop/cop/performance/redundant_merge.rb:135:28: C: [Correctable] Style/RedundantRegexpArgument: Use string "\n" as argument instead of regexp /\n/.
          new_source.gsub!(/\n/, padding)
                           ^^^^

116 files inspected, 2 offenses detected, 2 offenses autocorrectable
```

lib/rubocop/cop/performance/redundant_merge.rb

@@ -109,7 +109,7 @@ def correct_multiple_elements(corrector, node, parent, new_source)
             node = parent
           else
             padding = "\n#{leading_spaces(node)}"
-            new_source.gsub!(/\n/, padding)
+            new_source.gsub!("\n", padding)
           end
 
           corrector.replace(node, new_source)
@@ -132,7 +132,7 @@ def to_assignments(receiver, pairs)
         def rewrite_with_modifier(node, parent, new_source)
           indent = ' ' * configured_indentation_width
           padding = "\n#{indent + leading_spaces(node)}"
-          new_source.gsub!(/\n/, padding)
+          new_source.gsub!("\n", padding)
 
           format(WITH_MODIFIER_CORRECTION, keyword: parent.loc.keyword.source,
                                            condition: parent.condition.source,