Drop the json pin for Ruby 2.7.0 through 2.7.2 from the Gemfile

## Motivation and Context

json 3.0.2 loads on Ruby 2.7.0 through 2.7.2 again: it no longer forwards arguments after a leading parameter,
the syntax those patch levels could not parse, so the Gemfile no longer has to keep json below 3.0 there,
and the CI job pinned to 2.7.0, the minimum the gemspec supports, resolves json 3.0.2 like every other job.
The maintainers intend to raise json's own Ruby floor to 2.7.3 in the release after 3.0.2, so those patch levels
stay on 3.0.2 by json's gemspec from then on, and no pin of this Gemfile's own is needed for that either.

## How Has This Been Tested?

A fresh `bundle install` on Ruby 2.7.2 resolves json 3.0.2, `require "json"` loads it there,
and `bundle exec rake test` passes. Ruby 3.0 and later were never affected by the pin.

## Breaking Changes

None. The Gemfile is not part of the gem.

Author: koic

Gemfile

@@ -19,9 +19,6 @@ gem "activesupport"
 gem "debug" if RUBY_VERSION >= "3.1"
 # Avoid i18n 1.15.0, which breaks on Ruby 3.1 (ruby-i18n/i18n#735).
 gem "i18n", "!= 1.15.0"
-# FIXME: Drop this once a json release includes https://github.com/ruby/json/pull/1072. json 3.0.0 and 3.0.1
-# forward arguments after a leading parameter, syntax Ruby 2.7.3 was the first to parse, while allowing Ruby 2.7.0.
-gem "json", "< 3" if RUBY_VERSION < "2.7.3"
 gem "rake", "~> 13.0"
 gem "sorbet-static-and-runtime" if RUBY_VERSION >= "3.0"
 gem "yard", "~> 0.9"