[buildkite] Add mergify bot as allowed user

[mrodm]

Add mergify bot user as allowed user for buildkite PRs

[elasticmachine]

💔 Build Failed

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Build stats

• Start Time: 2023-08-17T11:06:44.773+0000

• Duration: 16 min 5 sec

Test stats 🧪

Test	Results
Failed	0
Passed	753
Skipped	1
Total	754

Steps errors

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test

• Took 2 min 15 sec . View more details here
• Description: make test-e2e

💚 Flaky test report

Tests succeeded.

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[mrodm]

/test

[jlind23]

buildkite test it

[jlind23]

Force merging this as this is blocked by ES 8.11 snapshot not being available and it blocks us while using mergify for backport.