[7.x](backport #28274) Filebeat: Change compatibility test stage to test against previous minor instead of 7.11

[mergify]

This is an automatic backport of pull request #28274 done by Mergify.

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[elasticmachine]

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• Start Time: 2021-10-07T20:19:31.291+0000

• Duration: 154 min 52 sec

• Commit: cd363a1

Test stats 🧪

Test	Results
Failed	0
Passed	54236
Skipped	5292
Total	59528

💚 Flaky test report

Tests succeeded.

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