Clarify README wording related to contributions

Also fixes the unquoted 3.0, which needs to be quoted to avoid truncation
Addresses a few Rubocop issues and an active_support require issue on Rails 7 w/Zeitwerk

.github/workflows/ci.yml

@@ -4,16 +4,16 @@ on: [push, pull_request]
 
 jobs:
   tests:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [2.4, 2.7, 3.0]
+        ruby: [2.4, 2.7, '3.0', 3.1, truffleruby-head]
 
     steps:
     - uses: actions/checkout@master
 
     - name: Sets up the Ruby version
-      uses: actions/setup-ruby@v1
+      uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
 

.rubocop.yml

@@ -42,6 +42,9 @@ Performance/TimesMap:
   Exclude:
     - 'spec/**/**.rb'
 
+Gemspec/RequiredRubyVersion:
+  Enabled: false
+
 # TODO: Fix these...
 Style/Documentation:
   Enabled: false

README.md

@@ -1,6 +1,10 @@
 # JSON:API Serialization Library
 
-## :warning: :construction: [At the moment, contributions are welcome only for v3](https://github.com/jsonapi-serializer/jsonapi-serializer/pull/141)! :construction: :warning:
+## :warning: :construction: v2 (the `master` branch) is in maintenance mode! :construction: :warning:
+
+We'll gladly accept bugfixes and security-related fixes for v2 (the `master` branch), but at this stage, contributions for new features/improvements are welcome only for v3. Please feel free to leave comments in the [v3 Pull Request](https://github.com/jsonapi-serializer/jsonapi-serializer/pull/141). 
+
+---
 
 A fast [JSON:API](https://jsonapi.org/) serializer for Ruby Objects.
 

jsonapi-serializer.gemspec

@@ -33,4 +33,5 @@ Gem::Specification.new do |gem|
   gem.add_development_dependency('rubocop-rspec')
   gem.add_development_dependency('simplecov')
   gem.add_development_dependency('sqlite3')
+  gem.metadata['rubygems_mfa_required'] = 'true'
 end

lib/fast_jsonapi/object_serializer.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require 'active_support'
 require 'active_support/time'
 require 'active_support/concern'
 require 'active_support/inflector'
@@ -133,9 +134,7 @@ module FastJsonapi
       def reflected_record_type
         return @reflected_record_type if defined?(@reflected_record_type)
 
-        @reflected_record_type ||= begin
+        @reflected_record_type ||= (name.split('::').last.chomp('Serializer').underscore.to_sym if name&.end_with?('Serializer'))
-          name.split('::').last.chomp('Serializer').underscore.to_sym if name&.end_with?('Serializer')
-        end
       end
 
       def set_key_transform(transform_name)

lib/fast_jsonapi/serialization_core.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require 'active_support'
 require 'active_support/concern'
 require 'digest/sha1'
 

lib/jsonapi/serializer/instrumentation.rb

@@ -1,3 +1,4 @@
+require 'active_support'
 require 'active_support/notifications'
 
 module JSONAPI

spec/fixtures/_user.rb

@@ -1,3 +1,4 @@
+require 'active_support'
 require 'active_support/cache'
 
 class User

spec/fixtures/actor.rb

@@ -1,3 +1,4 @@
+require 'active_support'
 require 'active_support/cache'
 require 'jsonapi/serializer/instrumentation'
 

spec/spec_helper.rb

@@ -6,6 +6,7 @@ SimpleCov.start do
 end
 SimpleCov.minimum_coverage 90
 
+require 'active_support'
 require 'active_support/core_ext/object/json'
 require 'jsonapi/serializer'
 require 'ffaker'