Deprecate arch flags in favor of multi-arch images

This commit deprecates the architecture flag (`--arch`) for the install
and bootstrap commands, in favor of the bundled multi-arch images that
will be available for the next MINOR range of GOTK controller releases.

Summary of changes:

* `*Arch` variables have been marked as deprecated for both commands.
* `-arm64` suffix is no longer selectively added to the image definition
  of a component's `Deployment`.
* `kubernetes.io/arch` node selector with the defined value has been
  removed from the components' `Deployment`s.
* `Arch` has been removed from the available `Options` in
  `manifestgen/install`.
* Documentation references have been changed to highlight existence
  of multi-arch images and supported architectures.

Signed-off-by: Hidde Beydals <hello@hidde.co>