Vitess 18: Remove deprecated k8stopo

[dbussink]

We've decided (see https://docs.google.com/document/d/1XLUdKePtj9aZD0E2Nlr3VRk1NIaSBgeIvXUj13y5CFk/edit?pli=1 under Vitess Monthly Meeting - 20 April 2023 to deprecate the k8stopo. In Vitess 17, using it will print a warning and with Vitess 18 the code will be removed. The main reasons are:

• No one is using this in production as far as we know.
• The code has existed for a long time but it was broken most of the time and not actually usable. That this was never reported and only found in Improve topo handling and add additional functionality #10906 (comment) when adding some additional unit tests. This also supports the previous point that no one is using this.
• Kubernetes as a dependency is heavy and anecdotally increases for example vttablet binary by ~40% in size. That's for something apparently unused. The dependency is also complex as it requires specific versioning that then causes issues for example with updating the https://github.com/planetscale/vitess-operator for specific Kubernetes versions. If that conflicts, we're in a problematic dependency conflict. Removing k8stopo allows for removing the entire Kubernetes dependency from Vitess itself, creating smaller binaries and easier to maintain code.
• Since no once is using it, there's no stakeholder to keep it updated if for example Kubernetes issues CVEs that force is to update.