Update suggested manifest for deploying the agent to GKE
Update suggested manifest for deploying the agent to GKE

Our GKE docs suggest a kubernetes Deployment with type "extensions/v1beta1/Deployment",
however Deployments moved to "apps/v1/Deployment" sometime ago and the
old namespace is [unavilable from kuberentes 1.16](https://kubernetes.io/blog/2019/07/18/api-deprecations-in-1-16/) (released mid 2019).

Our GKE docs in general are a bit dated and need an overhaul, but this
is a worthwhile quick-fix that'll save customers hitting a confusing
error.

I've also added matchLabels, because I think Deployment's are supposed
to have them.

This diff is mostly whitespace - I recmommend viewing with whitespace
changes hidden.

buildkite/docs,buildkite/docs,buildkite/docs,buildkite/docs