[automation] update elastic stack version for testing 8.4.0-aff0df12

[ghost]

What

Bump stack version with the latest one.

Further details

[start_time:Tue, 16 Aug 2022 12:15:36 GMT, release_branch:8.4, prefix:, end_time:Tue, 16 Aug 2022 19:17:51 GMT, manifest_version:2.1.0, version:8.4.0-SNAPSHOT, branch:8.4, build_id:8.4.0-aff0df12, build_duration_seconds:25335]

[elasticmachine]

💔 Build Failed

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Build stats

• Start Time: 2022-08-17T05:10:13.371+0000

• Duration: 26 min 40 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3
Skipped	0
Total	3

Steps errors

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Error signal

• Took 0 min 0 sec . View more details here
• Description: Error 'java.io.NotSerializableException: java.util.regex.Matcher'

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

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• /beats-tester : Run the installation tests with beats-tester.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[mergify]

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