Uses an alternative test method.

Should reduce failures like
[this](https://github.com/getsentry/sentry-javascript/actions/runs/9281632639/job/25538103763).

This adds a new E2E test `react-send-to-sentry` that is run optionally.

For now this is the same as the old `standard-frontend-react` test -
eventually we can/should update that test (and others) to stop sending
to sentry.

This test will run in a separate group that we do not block merging on
when it fails.

For now, there is one browser and one node test that checks that they
send events successfully to Sentry - IMHO that should cover stuff
decently for now. I also made the source maps debug ID test optional, as
that inherently sends to Sentry.

We can in follow ups get rid of all the event sending stuff from the
remaining E2E tests.

Part of #11910

No need for it anymore, and actually it is much simpler using fetch
because that does not throw when encountering a 404 error!

Instead of just testing to send to Sentry, this now tests the actual
payloads being sent.

This kind of builds on top of
#12259, where we
specifically test the event sending now.

I noticed that locally the node-integration-tests that check versions
fail, because in the SDK we use 18.18.0, we only check for "If
node.major >= 18", and then get an incorrect test because actually in
code we check the minor too.

Instead of adjust this, IMHO it's better to update this. This also never
failed on CI because there we do use latest 18 for the matrix tests...

This was not really needed, so we may as well remove it. We still use it
in package scripts, which is fine I'd say.

Note that this still does not "fix" the `yarn add` issue, but one step
at a time...

Co-authored-by: Luca Forstner <luca.forstner@sentry.io>

…extConfig.productionBrowserSourceMaps: true` is set (#12278)