[Heartbeat] Update agent-complete node version to 16.15.0

[andrewvc]

We've been on an old node version for a while, this is the latest LTS version.

Why is it important?

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

Pinging @elastic/uptime (Team:Uptime)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2022-05-20T21:16:36.155+0000

• Duration: 203 min 32 sec

Test stats 🧪

Test	Results
Failed	0
Passed	22288
Skipped	1933
Total	24221

💚 Flaky test report

Tests succeeded.

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[vigneshshanmugam]

LGTM

[justinkambic]

Discussed with @andrewvc offline and we agreed this can skip Post-FF testing