docs(gateway/builtin): update gateway docs after service tag deprecation in MeshGatewayInstance

[bartsmykla]

Update builtin gateway docs after deprecation of specifying kuma.io/service tag in MeshGatewayInstance

Closes https://github.com/kumahq/kuma-website/issues/1745

Rendered modified pages:

• https://deploy-preview-1748--kuma.netlify.app/docs/2.7.x/using-mesh/managing-ingress-traffic/builtin/
• https://deploy-preview-1748--kuma.netlify.app/docs/2.7.x/using-mesh/managing-ingress-traffic/builtin-k8s/
• https://deploy-preview-1748--kuma.netlify.app/docs/2.7.x/using-mesh/managing-ingress-traffic/builtin-listeners/

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