Add grpc and http2 listeners to gateway docs

[Dgotlieb]

Stating at Nomad version 1.2.0 grpc and http2 protocols are supported

The change affects gateway protocol docs

[hashicorp-cla]

All committers have signed the CLA.

[lgfa29]

Nice catch, thanks @Dgotlieb!

[Dgotlieb]

Thank you @lgfa29! I think at the same time we can also fix the sentence:

Note: If using http, preconfiguring a service-default in Consul to set the Protocol of the service to http is recommended.

To:

Note: If using http any non-tcp protocol, preconfiguring a service-default in Consul to set the Protocol of the service to the http desired protocol is recommended mandatory due to an open issue.

LMK if it seems correct and I will create a PR..

Thanks!

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