Actually use symfony matrix in tests

[mkilmanas]

By adding symfony/flex globally and passing SYMFONY_REQUIRE env param it ensures that all symfony packages are coming from the same symfony-version provided by the test matrix.

P.S. This also includes #96 as otherwise unable to build 7.0.* branch due to incompatibilities.

[unixslayer]

@mkilmanas please rebase onto current master branch

[coveralls]

Pull Request Test Coverage Report for Build 9260014815

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 94.118%

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Totals
Change from base Build 9259328194:	0.2%
Covered Lines:	528
Relevant Lines:	561

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💛 - Coveralls

[mkilmanas]

Had to exclude Symfony 7 vs PHP 8.1 combo from the matrix, because Symfony 7 itself requires php >= 8.2, so it was not installable by composer.

[unixslayer]

@mkilmanas awesome. Thanks.