Update protoconverter patch (envoyproxy#36290)

This serves to decouple protoconverter from the protobuf version it was
forked off of.

These changes should really be upstream, but upstream is experiencing
technical issues taking patches right now, so we'll carry them here
until those are resolved.

Risk Level: low
Testing: CI

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Signed-off-by: Alejandro R. Sedeño <[email protected]>