[6.0] Replace all joomla/string inflector code

[Hackwar]

Summary of Changes

The doctrine/inflector package is currently stuck on version 1.4.4, which right now is 4 years old. The current version 2.0.10 is mostly compatible - except for the Doctrine\Common\Inflector\Inflector class, which was deprecated and removed in 2.0. Unfortunately that class is the basis for the joomla/string packages inflector class. Since that base class is deprecated, this code removes all usages of the Joomla inflector package from our core code and instead uses the Doctrine package directly. The next step would be to update the package to the latest version, but that would break our joomla/string package, at least in our current situation. Fixing the string package to work with the newer code is left for someone else to work on...

The improvement here is, that it removes the usage of deprecated code and makes phpstan happy.

Testing Instructions

Codereview?

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[voronkovich]

Why don't we define an inflector as a service? If Joomla has a DI container, why don't we use it?