[Maps] Deprecate proxyElasticMapsServiceInMaps option

[thomasneirynck]

Deprecate the proxyElasticMapsServiceInMaps-option. When Kibana/EMS will support on-prem deployments, this setting will be much less relevant, since EMS can just be hosted (either as stand-alone or with reverse-proxy) behind a firewall.

Consider deprecating in 8.0.0.

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[thomasneirynck]

If on-prem will be the goto, we can then also deprioritze #69935 and #29825 accordingly

[thomasneirynck]

just log deprecation message until 8.0. Point to EMS oin prem as alternative

[thomasneirynck]

consider splitting in 2 issues:

• on prem support for Vega
• extract code from maps_legacy