Update to Symfony 5.3 & lock php minimum version

[gordalina]

Closes #180

[Seldaek]

Hey @gordalina long time no see ;)

Just a comment here it'd be probably safer to set "composer config platform.php 7.3.0” or whatever minimum php version is supported, that'll ensure composer update doesn't update packages to versions that don't support it, and would not require pinning the package here.

[gordalina]

Hey @Seldaek, it's been a while 👋!

Thank you for the heads up - this is definitely the right way to do it. Unfortunately, psr/container depends on 7.2 so even with the platform config it requires pinning the package.
Symfony 5.3 was just released and addresses the issue.