Allow JSON 3 on newer Rubies

[tagliala]

Summary

• remove the global JSON 2 development constraint now that multi_json 1.21.2 supports JSON 3 in its json_gem adapter
• retain json < 3 in the Rails 7.0, 7.1, and 7.2 appraisals because those still test Ruby 3.0/3.1, while multi_json 1.21.2 requires Ruby 3.2
• allow the default, Rails 8.0, Rails 8.1, and Rails edge bundles to resolve JSON 3

Oj compatibility

The Time serialization problem mentioned in the multi_json changelog is tracked upstream as ohler55/oj#1104. A proposed fix exists in ohler55/oj#1106, but it is not yet merged or released.

This does not affect ChronoModel: Oj is neither a dependency nor configured by this project, multi_json 1.21.2 prefers the JSON gem during automatic adapter selection, and ChronoModel uses MultiJson only for PostgreSQL view metadata composed of ordinary JSON primitives rather than Time values. Local verification selected MultiJSON::Adapters::JsonGem; Oj was not installed.

Verification

• bundle exec rubocop
• bundle exec rake with Rails 8.1, multi_json 1.21.2, and JSON 3.0.1: 509 examples, 0 failures
• BUNDLE_GEMFILE=/Users/geremia/dev/ifad/chronomodel/gemfiles/rails_8.0.gemfile bundle exec rake with JSON 3.0.1: 509 examples, 0 failures
• DDL metadata round trip through MultiJSON::Adapters::JsonGem with JSON 3.0.1
• fresh Ruby 3.1/Rails 7.1 resolution selects JSON 2.21.2 and multi_json 1.19.1, preserving the compatible legacy pair