Pin json below 3 in the Gemfile

A fresh bundle now resolves json 3, which is incompatible with Rails
before 8.1 and breaks test app setup with "unknown keyword:
quirks_mode". Solidus main pins it in its gemspecs; the 4.5 to 4.7
branches this extension is tested against do not, so pin it here.

Author: jarednorman

Gemfile

@@ -10,6 +10,9 @@ gem "solidus_backend", github: "solidusio/solidus", branch: branch
 rails_requirement_string = ENV.fetch("RAILS_VERSION", "7.0")
 gem "rails", "~> #{rails_requirement_string}"
 
+# json 3 is incompatible with Rails < 8.1.
+gem "json", "< 3", require: false
+
 # The solidus_frontend gem has been pulled out since v3.2
 if branch >= "v3.2"
   gem "solidus_frontend"