ref: Only use the envelop endpoint to send events to Sentry

[cleptric]

Problem Statement

The store API is considered deprecated and should not be used to send events anymore.

https://github.com/getsentry/sentry-php/blob/master/src/Transport/HttpTransport.php#L115-L123

Solution Brainstorm

As this will be a BC breaking change, we have to move this into a new major release.
We currently also lack the abstractions for the envelop header, but this will be added during the implementation of #1332.

We also noticed that we weren't very clear about the fact that the store API is deprecated and should update our docs (https://develop.sentry.dev/sdk/store/) accordingly.