Replace internal calls to helpers with StaticInflector

[donatj]

So while you have this warning, the library itself currently leans on these helpers, so much of the does not work without requiring vendor/icanboogie/inflector/lib/helpers.php

Warning

Since v3.0 the file with the helper functions is no longer included in the
autoload.
You need to include the file vendor/icanboogie/inflector/lib/helpers.php
in your composer.json if you want to continue using these functions.

So for instance

<?php

require 'vendor/autoload.php';

use ICanBoogie\Inflector;

echo Inflector::get('en')->pluralize('soup');

currently throws

Fatal error: Uncaught Error: Call to undefined function ICanBoogie\upcase() in /Users/jdonat/Sites/myon/vendor/icanboogie/inflector/lib/Inflections.php on line 255

This PR just changes all the call sites where the helpers are being used internally to use the static inflector instead of helpers

[donatj]

I also swapped ubuntu-20.04 with ubuntu-latest in your CI tests, as ubuntu-20.04 is no longer supported by GitHub actions. I can pull this change out if you are so inclined.

[coveralls]

coverage: 94.475%. remained the same
when pulling 41e1983 on donatj:fix/user-statics-internally
into efd39be on ICanBoogie:main.