[WIP] Symfony SDK version 5.0.0

[cleptric]

This will end up being a new major version of our Symfony SDK.

Remaining ToDos

• [ ] Add support for Dynamic Sampling https://github.com/getsentry/sentry-symfony/pull/661

TBD

New features

• [x] Add support for distributed tracing of the Symfony HTTP client requests https://github.com/getsentry/sentry-symfony/pull/606
• [x] Support logging the impersonator user https://github.com/getsentry/sentry-symfony/pull/647

Deprecations

• [x] Drop Symfony support below 4.4 https://github.com/getsentry/sentry-symfony/pull/643

Docs and product

• [ ] Enable Dynamic Sampling for the Symfony SDK
• [ ] Adjust docs for Symfony Dynamic Sampling

Misc

• [ ] Update README.md with now supported versions
• [ ] Update CHANGELOG.md with clear [deprecated] annotations and upgrade paths