Deprecate and remove support for a custom push URL

[Artur-]

Describe your motivation

There is no use case for setting a custom push URL. It is similar to the URL used for UIDL requests.

If you run a normal setup, the default always works.

If you want to use an external push server or similar, it won't work by just changing the URL

Describe the solution you'd like

There should be no setting for a custom push URL.

Before removing it it should show a message when used e.g.

You are defining a custom push URL. This is no longer supported as it is not needed for any use case and support will be removed in a future version.

[mshabarov]

This use case described here apparently needs a custom push URL API

[MarcinVaadin]

Can be closed as fix is available in https://github.com/vaadin/flow/pull/14638