chore(integrations/registry): remove deprecated kustomize features

[Stringls]

NOTE: Draft PR

Removing deprecated kustomize features as discussed here.

vars feature has not been removed as replacements cannot replace a var $() with the value patched outside the _base folder. I opened an issue in kustomize GitHub repository - link. I am awaiting for a response.

Also, besides removing deprecated kustomize features I also added missing files (e.g. kustomizeconfig.yaml to be able to use vars replacement in aws/ patch Deployment to specify a name of the secret with AWS credentials, etc.) and edited incorrect files.
I see the point why there were missing/incorrect files as on FluxCD website we suggest users to copy/paste the manifests and not to use those one stored in flux2 repository (perhaps we use these manifests somewhere else?). So it's fine if you'd like to revert the changes I made (adding new files).

I only updated registry-credentials-sync folder as there are already many changed files so you may wish to separate PRs so it's more convenient for you to review the PR. Please let me know your thoughts around that.