Drop support for Ruby 2.3 & 2.4

We no longer test on these rubies and this caused some [trouble][1] with the
Airbrake gem. We didn't catch this error becasue the Airbrake gem still tests on
2.3 & 2.4, but airbrake-ruby doesn't. We also never dropped support 2.3 & 2.4,
which left the whole situation in limbo.

```
5) Airbrake::Rack::Instrumentable.********_capture_timing when request store has a route attaches timing for a method with all arg types
     Failure/Error: params[:groups].merge!(timed_trace.spans)

     NoMethodError:
       undefined method `transform_values' for #<Hash:0x000055b7ee9f7300>
```

Since 2.3 & 2.4 are EOL for a long time (March 31, 2020 for 2.4), we can safely
stop supporting these rubies.

[1]: https://app.circleci.com/pipelines/github/airbrake/airbrake/215/workflows/d3f04866-05b9-4da6-ba15-b0c285eb0c23/jobs/9590

CHANGELOG.md

@@ -3,6 +3,11 @@ Airbrake Ruby Changelog
 
 ### master
 
+* Dropped support for Ruby 2.3
+  ([#663](https://github.com/airbrake/airbrake-ruby/pull/663))
+* Dropped support for Ruby 2.4
+  ([#663](https://github.com/airbrake/airbrake-ruby/pull/663))
+
 ### [v5.2.1][v5.2.1] (June 22, 2021)
 
 * Improved logging on closing notifiers

airbrake-ruby.gemspec

@@ -23,7 +23,7 @@ DESC
   s.files        = ['lib/airbrake-ruby.rb', *Dir.glob('lib/**/*')]
   s.test_files   = Dir.glob('spec/**/*')
 
-  s.required_ruby_version = '>= 2.3'
+  s.required_ruby_version = '>= 2.5'
 
   if defined?(JRuby)
     s.add_dependency 'rbtree-jruby', '~> 0.2'