Chore remove stable2603 upgrade working doc

[manuelmauro]

What does it do?

Deletes docs/cherry-picks/UPGRADE-stable2603.md.

This was a temporary working doc tracking the polkadot-sdk upgrade from stable2512 to stable2603. That upgrade shipped in #3746, and the doc's own header says to delete it once the upgrade lands.

What stays

Per the doc's drop checklist:

• polkadot-sdk-stable2603.md — the cherry-pick tracker for this cycle.
• polkadot-sdk-stable2512.md and polkadot-sdk-stable2506.md — kept for historical reference.

Notes

Docs-only change; nothing in the repo links to the deleted file.

Phase 5 (bridge maintenance: rebuild substrate-relay against stable2603, regenerate zombienet relay chain specs, run bridge integration tests) was still unchecked in the doc and is not carried over anywhere.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 3ecafe22-6c44-4463-8cd6-3ee7fe308a3f

📥 Commits

Reviewing files that changed from the base of the PR and between e991ea1 and c3c40f3.

📒 Files selected for processing (1)

• docs/cherry-picks/UPGRADE-stable2603.md

💤 Files with no reviewable changes (1)

• docs/cherry-picks/UPGRADE-stable2603.md

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📝 Walkthrough

Walkthrough

The pull request removes the file docs/cherry-picks/UPGRADE-stable2603.md, deleting the WIP upgrade plan document that tracked phases, branch/cherry-pick instructions, verification steps, and open risks for the polkadot-sdk to stable2603-1 upgrade.

Changes

Cohort / File(s)	Summary
Documentation removal docs/cherry-picks/UPGRADE-stable2603.md	Deleted the entire upgrade planning document, removing all tracked content.

Estimated code review effort: 1 (Trivial) | ~2 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes removing the stable2603 upgrade working doc.
Description check	✅ Passed	The description is directly related to deleting the upgrade doc and explains the scope correctly.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore-remove-stable2603-upgrade-doc

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