Use RuboCop Performance to 1.20

```console
$ bundle exec rubocop
(snip)

lib/rubocop/cop/exclude_limit.rb:11:21: C: [Correctable] Performance/StringIdentifierArgument:
Use :"#{method_name}=" instead of "#{method_name}=".
      define_method("#{method_name}=") do |value|
                    ^^^^^^^^^^^^^^^^^
lib/rubocop/cop/style/conditional_assignment.rb:236:25: C: [Correctable] Performance/StringIdentifierArgument:
Use :"on_#{type}" instead of "on_#{type}".
          define_method "on_#{type}" do |node|
                        ^^^^^^^^^^^^
lib/rubocop/cop/style/numeric_literal_prefix.rb:65:32: C: [Correctable] Performance/StringIdentifierArgument:
Use :"#{literal_type(node).upcase}_MSG" instead of "#{literal_type(node).upcase}_MSG".
          self.class.const_get("#{literal_type(node).upcase}_MSG")
                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
lib/rubocop/lsp/routes.rb:20:23: C: [Correctable] Performance/StringIdentifierArgument:
Use :"handle_#{name}" instead of "handle_#{name}".
        define_method("handle_#{name}", &block)
                      ^^^^^^^^^^^^^^^^

1527 files inspected, 4 offenses detected, 4 offenses autocorrectable
```

Gemfile

@@ -10,7 +10,7 @@ gem 'bundler', '>= 1.15.0', '< 3.0'
 gem 'memory_profiler', platform: :mri
 gem 'rake', '~> 13.0'
 gem 'rspec', '~> 3.7'
-gem 'rubocop-performance', '~> 1.19.0'
+gem 'rubocop-performance', '~> 1.20.0'
 gem 'rubocop-rake', '~> 0.6.0'
 gem 'rubocop-rspec', '~> 2.25.0'
 # Workaround for cc-test-reporter with SimpleCov 0.18.

lib/rubocop/cop/exclude_limit.rb

@@ -8,7 +8,7 @@ module ExcludeLimit
     # The parameter name given is transformed into a method name (eg. `Max`
     # becomes `self.max=` and `MinDigits` becomes `self.min_digits=`).
     def exclude_limit(parameter_name, method_name: transform(parameter_name))
-      define_method("#{method_name}=") do |value|
+      define_method(:"#{method_name}=") do |value|
         cfg = config_to_allow_offenses
         cfg[:exclude_limit] ||= {}
         current_max = cfg[:exclude_limit][parameter_name]

lib/rubocop/cop/style/conditional_assignment.rb

@@ -233,7 +233,7 @@ class ConditionalAssignment < Base
         PATTERN
 
         ASSIGNMENT_TYPES.each do |type|
-          define_method "on_#{type}" do |node|
+          define_method :"on_#{type}" do |node|
             return if part_of_ignored_node?(node)
             return if node.parent&.shorthand_asgn?
 

lib/rubocop/cop/style/numeric_literal_prefix.rb

@@ -62,7 +62,7 @@ def on_int(node)
         private
 
         def message(node)
-          self.class.const_get("#{literal_type(node).upcase}_MSG")
+          self.class.const_get(:"#{literal_type(node).upcase}_MSG")
         end
 
         def literal_type(node)

lib/rubocop/lsp/routes.rb

@@ -17,7 +17,7 @@ module Lsp
     # @api private
     class Routes
       def self.handle(name, &block)
-        define_method("handle_#{name}", &block)
+        define_method(:"handle_#{name}", &block)
       end
 
       private_class_method :handle