eliminate code duplication between (legacy) output proxy and doppler proxy

- delete output proxy
- add legacy proxy which wraps doppler proxy

[#76285060]

Signed-off-by: Johannes Petzold <jpetzold@pivotal.io>
Revert "bump loggregatorlib"

This reverts commit 98021e566ed1ed84813c0e511914c79b477aca8e.