Specify testing context of Ruby 2.7 for Performance/BindCall

This PR prepares for rubocop#112 and makes the `rake spec` successful
with RuboCop 0.71, which is supported by RuboCop Performance 1.6.

This is a workaround until the CI is ready for RuboCop Performance 1.7.
RuboCop Performance 1.7 will drop some older RuboCop support versions.

spec/rubocop/cop/performance/bind_call_spec.rb

@@ -1,6 +1,10 @@
 # frozen_string_literal: true
 
 RSpec.describe RuboCop::Cop::Performance::BindCall, :config do
+  # TODO: The following is no longer required when RuboCop 0.78 or lower support will be dropped.
+  # /~https://github.com/rubocop-hq/rubocop/pull/7605
+  let(:ruby_version) { 2.7 }
+
   subject(:cop) { described_class.new(config) }
 
   context 'TargetRubyVersion <= 2.6', :ruby26 do