Deprecate and remove the Spotinst integration

## Summary

Proposed schedule:

- **kOps 1.37:** Deprecate the Spotinst integration.
- **kOps 1.38:** Block creation of new Spotinst clusters.
- **kOps 1.39:** Remove the integration.

In December 2022, kOps added a Help Wanted request for a maintainer and warned that the integration could be phased out. Spot responded in May 2023 (#15357) and contributed through November 2024 (#16931), but the integration remains alpha, feature-gated, and unmaintained.

## Why

- No integration-specific maintainer activity since November 2024.
- The Spotinst OWNERS files have no approvers or reviewers.
- `spotinst-sdk-go` is pinned at v1.372.0, 43 releases behind, and explicitly excluded from automated dependency updates.
- There are no dedicated unit, integration, or e2e tests.
- The integration contains about 7,100 lines of code plus 26,000 vendored SDK lines and is coupled into shared AWS code.

Removal must prevent `kops update cluster` from silently creating duplicate ASGs alongside a live Ocean or Elastigroup cluster. The Spot resource deletion path must remain available for one additional release so existing clusters can still be deleted safely.

**Production users: please comment with your kOps version and the Spot features you depend on.**

Anyone willing to maintain the integration should reply on this issue.

/kind cleanup
/area provider/spotinst

Trace the Spotinst provider through the shared AWS code and the `kops update cluster` entry point; review the Spotinst OWNERS files and the `spotinst-sdk-go` dependency. Confirm the staged 1.37/1.38/1.39 plan, including retaining Spot resource deletion for one release, and verify that updates cannot create duplicate ASGs.

Written by the indexing model from the issue text.