Added support for Symfony 5.4

[maruf-paysera]

Summary

This MR:

1. adds support to Symfony 5.4
2. Drop support for old php versions (below php 5.5)
3. Drop support for old Symfony versions (backward compatibility with old Symfony versions is not possible due to symfony changed the authentication mechanism since version 5)

Test

1. ./vendor/bin/phpunit

[maruf-paysera]

@mSprunskas please review

[mSprunskas]

Does not work on sf 4, please test code on both sf4 and sf5

[maruf-paysera]

Does not work on sf 4, please test code on both sf4 and sf5

@mSprunskas tested on both sf4 and sf5, works as expected