Drop Symfony 4 support, fix deprecation for Extension namespace, and some chores

What's Changed

• drop reference to .env.dist by @tacman in #435
• drop reference to Symfony 3 by @tacman in #433
• [dx] keep tooling in one place by @jrushlow in #445
• drop Symfony 4.x support by @jrushlow in #443
• [ci] php-cs-fixer it up by @jrushlow in #448
• Fix Symfony 7.1 deprecation for Extension by @Evgeny1973 in #449
• Fix CS by @bocharsky-bw in #450

New Contributors

• @Evgeny1973 made their first contribution in #449

Full Changelog: v2.18.1...v2.18.2