[CI] use obltGitHubComments

[v1v]

What does this PR do?

Use https://github.com/elastic/apm-pipeline-library/blob/master/vars/obltGitHubComments.txt

Why is it important?

Avoid hardcoded strings and easy to normalise the GitHub comments in all the oblt projects

You can also see in the GitHub comment that there is a specific section for some of those GitHub comments called 🤖 GitHub comments

[elasticmachine]

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Build stats

• Start Time: 2021-09-22T09:04:29.424+0000

• Duration: 31 min 42 sec

• Commit: d380339

Test stats 🧪

Test	Results
Failed	0
Passed	433
Skipped	5
Total	438

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