[deps] upgrade to cenkalti/backoff/v5

[ultrotter]

The newest version is /v7, but we already have v5 in the codebase via OTel, so upgrading to v5 allows us to depend into just one copy of this library.

Pull Request Checklist

Please check all the applicable boxes.

• Is this a breaking change?
  • My changes do not break the existing cluster messages
  • My changes do not break the existing api
• I have signed-off my commits
• I will follow best practices for contributing to this project

Which user-facing changes does this PR introduce?

NONE

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ℹ️ Recent review info

⚙️ Run configuration

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Review profile: CHILL

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Run ID: f933a68b-e549-49f2-a30c-185ee4b8cb98

📥 Commits

Reviewing files that changed from the base of the PR and between 963ed72 and 8c2bee9.

⛔ Files ignored due to path filters (1)

• go.sum is excluded by !**/*.sum

📒 Files selected for processing (2)

• go.mod
• notify/retry_stage.go

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📝 Walkthrough

Walkthrough

This PR upgrades the github.com/cenkalti/backoff dependency from v4 to v5 in go.mod, and updates notify/retry_stage.go to import backoff/v5, removing the explicit MaxElapsedTime override and relying on context cancellation to stop retries.

Changes

Backoff Dependency Upgrade

Layer / File(s)	Summary
Dependency version bump go.mod	Direct requirement changed from backoff/v4 v4.3.0 to backoff/v5 v5.0.3; removed the redundant indirect backoff/v5 entry.
Retry stage backoff usage update notify/retry_stage.go	Import switched to backoff/v5; removed MaxElapsedTime = 0 override and added a comment explaining that v5's exponential backoff never signals Stop, so retries continue until context cancellation.

Estimated code review effort: 2 (Simple) | ~10 minutes

Related PRs: None identified from the provided context.

Suggested labels: dependencies, go

Suggested reviewers: None identified from the provided context.

🐰 A backoff hopped from four to five,
No more a fixed time to survive,
The context now holds the reins,
Retries persist through all the lanes,
A tiny bump keeps alerts alive.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the dependency upgrade to cenkalti/backoff/v5.
Description check	✅ Passed	The description is related to the dependency upgrade and explains the motivation for switching to v5.
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

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