Cherry-pick #23955 to 7.x: Update Golang to 1.15.8

[michalpristas]

Cherry-pick of PR #23955 to 7.x branch. Original message:

What does this PR do?

This PR updates golang to 1.15.8

Why is it important?

.8 fixes some critical bugs

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.

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Pinging @elastic/agent (Team:Agent)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: Pull request #24008 opened

• Start Time: 2021-02-11T15:59:29.505+0000

• Duration: 100 min 3 sec

• Commit: 086eb5e

Test stats 🧪

Test	Results
Failed	0
Passed	45183
Skipped	4672
Total	49855

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Steps errors

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x-pack/filebeat-Lint - Install Go/Mage/Python/Docker/Terraform 1.15.8

• Took 1 min 36 sec . View more details on here
• Description: .ci/scripts/install-tools.sh

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Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	45183
Skipped	4672
Total	49855

[blakerouse]

Backport looks good.