http2: improve compatibility with http/1

When using the compatibility API the connection header is from now on
ignored instead of throwing an `ERR_HTTP2_INVALID_CONNECTION_HEADERS`
error.
This logs a warning in such case to notify the user about the ignored
header.

PR-URL: https://github.com/nodejs/node/pull/23908
Fixes: https://github.com/nodejs/node/issues/23748


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>