Suppress warning: constant Net::HTTPServerException is deprecated

This PR suppresses the following warning when Ruby 2.6 or higher.

```console
% ruby -v
ruby 3.0.2p107 (2021-07-07 revision 0db68f0233) [x86_64-darwin19]
% RUBYOPT=-w be rspec spec/rubocop/remote_config_spec.rb
(snip)

Randomized with seed 52290
/Users/koic/src/github.com/rubocop/rubocop/spec/rubocop/remote_config_spec.rb:80:
warning: constant Net::HTTPServerException is deprecated
/Users/koic/src/github.com/rubocop/rubocop/spec/rubocop/remote_config_spec.rb:125:
warning: constant Net::HTTPServerException is deprecated
```

spec/rubocop/remote_config_spec.rb

@@ -75,9 +75,11 @@
         end
 
         it 'raises error' do
+          exception = RUBY_VERSION >= '2.6' ? Net::HTTPClientException : Net::HTTPServerException
+
           expect do
             remote_config
-          end.to raise_error(Net::HTTPServerException,
+          end.to raise_error(exception,
                              '404 "" while downloading remote config file http://example.com/rubocop.yml')
         end
       end
@@ -120,9 +122,11 @@
         end
 
         it 'raises error' do
+          exception = RUBY_VERSION >= '2.6' ? Net::HTTPClientException : Net::HTTPServerException
+
           expect do
             remote_config
-          end.to raise_error(Net::HTTPServerException,
+          end.to raise_error(exception,
                              '404 "" while downloading remote config file http://example.com/rubocop.yml')
         end
       end