Add CI; drop EOL PHP and Symfony versions

[andrewmy]

• Add CI which runs existing tests;
• Do not add EOL PHP and Symfony versions to the build matrix;
• Drop the EOL versions from composer.json.

[andrewmy]

@Nyholm please allow running the workflow here

[Nyholm]

Wohoo.

It almost worked. https://github.com/sroze/messenger-enqueue-transport/actions/runs/7246392911/job/19738239635#step:8:27