[7.15](backport #28530) Fix formatting of mapStateJSON and layerListJSON in dashboard assets

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This is an automatic backport of pull request #28530 done by Mergify.

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Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

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• Start Time: 2021-10-21T12:40:02.636+0000

• Duration: 176 min 45 sec

• Commit: be75aec

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Test	Results
Failed	0
Passed	53143
Skipped	5289
Total	58432

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