Update json gem to v2.5.1

What does this MR do and why?

In preparation for the Ruby 3 upgrade, we should update json to match the minimum gem version as specified by the default gem:

https://stdgems.org/json/#gem-version

Which for Ruby 3.0 is 2.5.1.

This will otherwise lead to errors, since rubygems will encounter conflicting gemspecs during require.

See omnibus-gitlab#6286 (closed)

Changelog

2020-12-22 (2.5.1)

- Restore the compatibility for constants of JSON class.

2020-12-22 (2.5.0)

- Ready to Ractor-safe at Ruby 3.0.

2020-12-17 (2.4.1)

- Restore version.rb with 2.4.1

2020-12-15 (2.4.0)

- Implement a freeze: parser option #447
- Fix an issue with generate_pretty and empty objects in the Ruby and Java implementations #449
- Fix JSON.load_file doc #448
- Fix pure parser with unclosed arrays / objects #425
- bundle the LICENSE file in the gem #444
- Add an option to escape forward slash character #405
- RDoc for JSON #439 #446 #442 #434 #433 #430

https://github.com/flori/json/blob/master/CHANGES.md#2020-12-22-251

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