Update to main branch when using beats-tester

[simitt]

Motivation/summary

https://github.com/elastic/beats-tester has switched to main branch. APM Server needs to update dependencies accordingly.

[mergify]

This pull request does not have a backport label. Could you fix it @simitt? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.x is the label to automatically backport to the 7.x branch.
• backport-7./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[apmmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2021-12-21T00:29:30.756+0000

• Duration: 43 min 50 sec

• Commit: 64e4c0e

Test stats 🧪

Test	Results
Failed	0
Passed	5616
Skipped	19
Total	5635

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