Deprecate and remove the Spotinst integration

[hakman]

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

[hakman]

CC @rifelpet @amends @justinsb