Remove ECE stackpack 9.5.1 and document it as a known issue

[beiske]

Part of permanently removing Elastic Stack 9.5.1 from Cloud (INC-3460 — same must_not doc-values-only query-correctness defect that affects 9.5.0).

Summary

• Remove 9.5.1 from the ECE 4.0 available stack packs / offline Docker image lists:
  • deploy-manage/deploy/cloud-enterprise/ece-install-offline-images.md
  • deploy-manage/deploy/cloud-enterprise/manage-elastic-stack-versions.md
• Extend the existing {{ecloud}} known-issue entry so it covers 9.5.0 and 9.5.1 in deploy-manage/deploy/elastic-cloud/restrictions-known-problems.md.

Follows the permanently-remove-a-stackpack-version guide. The stackpack pipeline block lives in elastic/cloud-assets#2248.

Note: per the guide, we currently cannot delete the zip from the S3 bucket, so it is intentionally left there.

(AI-authored via Cursor 🤖)

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🔍 Preview links for changed docs

• deploy-manage/deploy/cloud-enterprise/ece-install-offline-images.md
• deploy-manage/deploy/cloud-enterprise/manage-elastic-stack-versions.md
• deploy-manage/deploy/elastic-cloud/restrictions-known-problems.md

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✅ Elastic Docs Style Checker (Vale)

No issues found on modified lines!

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The Vale linter checks documentation changes against the Elastic Docs style guide. To use Vale locally or report issues, refer to Elastic style guide for Vale.

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Docs review summary

Focus areas

• Style and clarity: No issues introduced by this diff. The extended known-issue sentence on restrictions-known-problems.md (line 42) follows the same pattern as the existing sentence and reads clearly.
• Jargon: None found in the changed lines.
• Frontmatter and applies_to: Not affected — no frontmatter changes in this PR.
• Content type fit: No mismatch — troubleshooting/reference table edits fit the existing page types.
• Contradictions: No contradictions found. The version-removal edits are internally consistent across the two ECE stackpack files, and the known-issue update correctly extends the existing 9.5.0 entry to also cover 9.5.1.
• Parent issue satisfaction: Not applicable — no linked issue; the PR body's stated goal (remove 9.5.1 stackpack, document as known issue) is fully reflected in the diff.

Notes

• Pre-existing Vale findings (e.g., mapped_pages edits, "or higher" vs "or later") fall outside the lines changed by this PR and are not related to this change, so they weren't flagged here.

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