[automation] update elastic stack version for testing 7.15.3-c7a19acd

[ghost]

What

Bump stack version with the latest one.

Further details

[start_time:Mon, 29 Nov 2021 17:25:42 GMT, release_branch:7.15, prefix:, end_time:Mon, 29 Nov 2021 23:15:32 GMT, manifest_version:2.0.0, version:7.15.3-SNAPSHOT, branch:7.15, build_id:7.15.3-c7a19acd, build_duration_seconds:20990]

[elasticmachine]

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Build stats

• Start Time: 2021-11-30T05:13:08.485+0000

• Duration: 5 min 47 sec

• Commit: 761782d

Test stats 🧪

Test	Results
Failed	0
Passed	144
Skipped	0
Total	144

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Tests succeeded.

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