Keep json below 3.0 on Ruby 2.7.0 through 2.7.2 in the Gemfile

## Motivation and Context

json 3.0 declares Ruby 2.7 as its floor, but its `load_file` forwards arguments after a leading parameter,
`def load_file(filespec, ...)`, syntax Ruby 2.7.3 was the first to parse. On Ruby 2.7.0 through 2.7.2, requiring
json 3.0.1 is a `SyntaxError`, so `require "mcp"` fails before any test runs, and the CI job pinned to 2.7.0,
the minimum the gemspec supports, fails the whole suite at load time.

The Gemfile keeps json below 3.0 on those three patch levels only. Later 2.7 releases and Ruby 3 parse the syntax
and stay on whatever json the resolver picks.
The line carries a FIXME to drop it once a json release includes ruby/json#1072, whichever version that turns out to be;
excluding only the two broken releases would silently trust a 3.0.2 that may not carry the fix.

## How Has This Been Tested?

`gem "json", "3.0.1"; require "json"` raises the `SyntaxError` on Ruby 2.7.0 and loads on 2.7.3 and 2.7.8.
With the pin, a fresh `bundle install` on Ruby 2.7.2 resolves json 2.x and `bundle exec rake test` passes.

## Breaking Changes

None. The Gemfile is not part of the gem.

Author: koic

Gemfile

@@ -19,6 +19,9 @@ 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"