Add Symfony 7.0 to reqs and CI, drop 6.2 from CI; drop PHP 7.4

[andrewmy]

Let's install this on modern apps too!

Prod changes:

• Add Symfony ^7.0 to composer requirements;
• Add Symfony 7.0.* to CI;
• Drop Symfony 6.2.* (EOL) from CI;
• Drop PHP 7.4 (EOL) from composer reqs and CI.

Got some random AMQP type errors on PHP 7.4 and decided to drop it rather than fix.

PHP 8.0 is also EOL but not bothering us much for now.

Dev changes:

• Bump localstack to v3 to accommodate for new AWS SDK which now only does SQS-JSON instead of XML;
• Bump PHPUnit to 9.6.15;
• Fix PHP 8.2 builds, the matrix was messed up 🙈

[TechComBiz]

@andrewmy What is your planning with this PR?
I was started to add Symfony 7 to composer.json (#1331). But that was more impact than I expected.

[andrewmy]

@TechComBiz I see a weird failure on one SnsQs test on some matrix branches, thinking how to debug this