Use the DI container for creating transport factories

[mtangoo]

Q	A
Is bugfix?	✔️/❌
New feature?	✔️/❌
Breaks BC?	✔️/❌
Fixed issues

[what-the-diff]

PR Summary

• Updated CHANGELOG for new version
  The CHANGELOG has been updated for the new version (4.0.0). This comes with several enhancements such as using Dependency Injection (DI) container for creating factories, incorporating 'logger proxy' as a dependency, and ending support for outdated PHP versions 7.4 and 8.0.

• Enhanced README with DI container usage
  The README documentation has been updated to include details about how to use the Dependency Injection (DI) container.

• Upgraded dependencies in composer.json
  The project's dependencies on 'symfony/mailer' and 'symfony/mime' have been upgraded to version 7.0.

• Refactor of Mailer class in source code
  The 'Mailer' class has been refactored for more efficient operations. There are several updates, including the use of 'DynamicLogger' (when available), enhancing the 'getTransportFactory()' method, and the inclusion of a new 'sendMessage()' method.

• New DI Container test
  A new test to manage the Dependency Injection container usage has been added to our suite of validation tests.

[samdark]

Should go into 4.0.0 ↓