CI: Ruby 2.4 not supported by test-unit-ruby-core

Your RubyGems version (2.6.14.4) has a bug that prevents `required_ruby_version` from working for Bundler. Any scripts that use `gem install bundler` will break as soon as Bundler drops support for your Ruby version. Please upgrade RubyGems to avoid future breakage and silence this warning by running `gem update --system 3.2.3`
/home/runner/work/forwardable/forwardable/vendor/bundle/ruby/2.4.0/gems/test-unit-ruby-core-1.0.15/lib/core_assertions.rb:390: warning: possibly useless use of a variable in void context
/home/runner/work/forwardable/forwardable/vendor/bundle/ruby/2.4.0/gems/test-unit-ruby-core-1.0.15/lib/core_assertions.rb:404: warning: mismatched indentations at 'end' with 'def' at 342
/home/runner/work/forwardable/forwardable/vendor/bundle/ruby/2.4.0/gems/test-unit-ruby-core-1.0.15/lib/core_assertions.rb:413: warning: mismatched indentations at 'end' with 'module' at 74
/home/runner/work/forwardable/forwardable/vendor/bundle/ruby/2.4.0/gems/test-unit-ruby-core-1.0.15/lib/core_assertions.rb:1030: warning: mismatched indentations at 'end' with 'module' at 41
/home/runner/work/forwardable/forwardable/vendor/bundle/ruby/2.4.0/gems/test-unit-ruby-core-1.0.15/lib/core_assertions.rb:1031: warning: mismatched indentations at 'end' with 'module' at 3
/opt/hostedtoolcache/Ruby/2.4.10/x64/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:133:in `require': /home/runner/work/forwardable/forwardable/vendor/bundle/ruby/2.4.0/gems/test-unit-ruby-core-1.0.15/lib/core_assertions.rb:390: syntax error, unexpected keyword_rescue, expecting keyword_end (SyntaxError)
        rescue => marshal_error
              ^
/home/runner/work/forwardable/forwardable/vendor/bundle/ruby/2.4.0/gems/test-unit-ruby-core-1.0.15/lib/core_assertions.rb:393: syntax error, unexpected keyword_else, expecting keyword_end
        else
            ^
/home/runner/work/forwardable/forwardable/vendor/bundle/ruby/2.4.0/gems/test-unit-ruby-core-1.0.15/lib/core_assertions.rb:1032: syntax error, unexpected keyword_end, expecting end-of-input
	from /opt/hostedtoolcache/Ruby/2.4.10/x64/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:133:in `rescue in require'
	from /opt/hostedtoolcache/Ruby/2.4.10/x64/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:39:in `require'
	from /home/runner/work/forwardable/forwardable/test/lib/helper.rb:2:in `<top (required)>'
	from /opt/hostedtoolcache/Ruby/2.4.10/x64/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /opt/hostedtoolcache/Ruby/2.4.10/x64/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
rake aborted!
Command failed with status (1)