T7748: conflict check workflow updated to use marketplace action along with increased wait/retry

[kumvijaya]

Change summary

The conflict check workflow was showing inconsistent behavior in detecting merge conflicts — sometimes not reporting the correct conflict status.

To improve reliability and maintainability, the workflow was updated to use a marketplace action with increased wait and retry settings. This ensures GitHub has enough time to compute the correct mergeable status before the workflow makes a decision.

Note: GitHub may return an unknown mergeable state immediately after changes in the base branch.
Without retries, this caused false negatives (reporting no conflicts when they actually existed).
By adding retries and wait intervals, the workflow should consistently report the correct status.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

https://vyos.dev/T7748

Related PR(s)

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

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