Deprecate `init` context manager

See below:

> `init` is generally one per process anyway. In case people want DSN isolation, multiple clients is the way to go now. So I really don't see the use case for this. 
> Let's deprecate and remove.

_Originally posted by @sl0thentr0py in https://github.com/getsentry/sentry-python/pull/3276#discussion_r1675776684_
            