Bump minimum required version of the core SDK to 3.9

[cleptric]

This is needed to support DS in the Symfony SDK.

[stayallive]

Shouldn't Symfony require sentry/sentry with it's own constraint instead of bumping the SDK just for Symfony (like we do in Laravel) 😅 (obviously this is also fine but could be simpler?)

Edit: Which you also did: getsentry/sentry-symfony#661 (comment). Why shouldn't we keep that constraint at the Symfony side? Wouldn't that make it simpler and we can just release a sentry/sdk when we really need people to upgrade or when we release a major version.

[cleptric]

We can continue the discussion over at getsentry/sentry-symfony#643. I don't see why we shouldn't do the same as in the Laravel SDK either.