build: avoid Rails 7 on MRI 2.4.

Author: pat

Gemfile

@@ -7,6 +7,8 @@ gemspec
 gem 'mysql2', '~> 0.5.0',  :platform => :ruby
 gem 'pg',     '~> 0.18.4', :platform => :ruby
 
+gem 'activerecord', '< 7' if RUBY_VERSION.to_f <= 2.4
+
 if RUBY_PLATFORM == 'java'
   gem 'jdbc-mysql',                          '5.1.35',    :platform => :jruby
   gem 'activerecord-jdbcmysql-adapter',      '>= 1.3.23', :platform => :jruby