Implement server-side apply across Flux projects

[stefanprodan]

When we started Flux v2, we've set a goal to stop relying on 3rd party binaries for core features. While we've successfully replaced the Git CLI shell execs with Go libraries (go-git, git2go) and C libraries (libgit2, libssh2), the kustomize CLI with Go libraries (kustomize/api, kustomize/kyaml), we're still depending on the kubectl CLI for the 3-way-merge apply feature. With Kubernetes server-side apply being promoted to GA, we can finally get rid of kubectl and drive the reconciliation using exclusively the controller-runtime Go client.

Using server-side apply means that the minimum supported version of Kubernetes is:

Kubernetes version	Minimum required
v1.16	>= 1.16.11
v1.17	>= 1.17.7
v1.18	>= 1.18.4
v1.19 and later	>= 1.19.0

Coding tasks

• PoC for replacing kubectl with controller-runtime client and Kubernetes server-side apply
  v1: Reimplement reconciliation with server-side apply stefanprodan/kustomizer#17
• Implement a thin wrapper over Kubernetes server-side apply to be shared between all Flux projects
  Add server-side apply package pkg#152
• Rewrite the kustomize-controller reconciler using the SSA package
  [RFC] Server-side reconciliation for the v1beta2 API kustomize-controller#426
• Replace kubectl usage in Flux CLI with the SSA package
  Replace kubectl with Go server-side apply #1885
• Update the Kubernetes version semver ranges in flux check according to the above table
  Update Kubernetes version minimum requirements #1899
• Update the kustomize-controller API to v1beta2 and deprecate the validation arg
  Update kustomize-controller API to v1beta2 #1900
• Remove kubectl dependency from the Homebrew formula and AUR packages
  Remove kubectl dependency from brew and aur pkgs  #1901

Documentation tasks

• Update the get start guide and set the target namespace
• Update the kustomize-controller API docs to v1beta2
• Update all Kustomization samples to v1beta2 and remove the .spec.validation field
• Update the minimum required version of Kubernetes in install docs

Note that the docs PRs must be merged after flux2 v0.18.0 release.