Bump RuboCop to 0.51.0

## Summary

RuboCop 0.51.0 was released.
https://github.com/bbatsov/rubocop/releases/tag/v0.51.0

And rubocop-0-51 channel is available in Code Climate.
https://github.com/codeclimate/codeclimate-rubocop/issues/109

This PR will bump RuboCop to 0.51.0 and fixes the following new
offenses.

```console
% bundle exec rubocop
Inspecting 2358 files

(snip)

Offenses:

actionpack/lib/action_controller/metal/http_authentication.rb:251:59: C:
Prefer double-quoted strings unless you need single quotes to avoid
extra backslashes for escaping.
          [key.strip, value.to_s.gsub(/^"|"$/, "").delete('\'')]
                                                          ^^^^
activesupport/test/core_ext/load_error_test.rb:8:39: C: Prefer
double-quoted strings unless you need single quotes to avoid extra
backslashes for escaping.
    assert_raise(LoadError) { require 'no_this_file_don\'t_exist' }
                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^

2358 files inspected, 2 offenses detected
```

1 files changed, 1 insertions, 1 deletions

diff --git a/actionpack/lib/action_controller/metal/http_authentication.rb b/actionpack/lib/action_controller/metal/http_authentication.rb
index 08d9b094f3..0c8132684a 100644
--- a/actionpack/lib/action_controller/metal/http_authentication.rb
+++ b/actionpack/lib/action_controller/metal/http_authentication.rb
@@ -248,7 +248,7 @@ module ActionController
       def decode_credentials(header)
         ActiveSupport::HashWithIndifferentAccess[header.to_s.gsub(/^Digest\s+/, "").split(",").map do |pair|
           key, value = pair.split("=", 2)
-          [key.strip, value.to_s.gsub(/^"|"$/, "").delete('\'')]
+          [key.strip, value.to_s.gsub(/^"|"$/, "").delete("'")]
         end]
       end