Release notes for 4.0

• #66 Adds the final keyword to all classes preventing subclassing
• #70 Introduces native types where they were previously missing. Most of these changes will not cause any impact now that the classes are final.
• #71 removes the deprecated ImplicitMethodsIntegrationTest - this abstract test case was renamed for PHPUnit compatibility to AbstractImplicitMethodsIntegrationTest. This change will only affect integration tests for Router implementations
• #68 changes the constructor signature of ImplicitHeadMiddleware - it now only accepts a PSR-17 StreamFactoryInterface where previously, it also accepted an arbitrary callable.
• #69 changes the constructor signatures of MethodNotAllowedMiddleware and ImplicitOptionsMiddleware. Previously, both of these middleware accepted a callable and a PSR-17 ResponseFactoryInterface. The ability to pass a callable has been removed
• Some internal, and now unused traits and classes have been removed. As these were not part of the public API, their removal should not cause any problems.