Use mapstr.M instead of common.MapStr

[kvch]

What does this PR do?

This PR adopts mapstr.M and gets rid of common.MapStr from libbeat.

Why is it important?

We are removing common parts from libbeat in favour of elastic-agent-libs.

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]

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• Start Time: 2022-04-28T09:46:44.531+0000

• Duration: 142 min 31 sec

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Test	Results
Failed	0
Passed	24634
Skipped	2142
Total	26776

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[rdner]

There are some failures on the CI (not linting-related).

[rdner]

It's not over yet :)

[kvch]

To decrease the amount of changes, the documentation fixes will come in a new PR.

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b use-mapstr-from-libs upstream/use-mapstr-from-libs
git merge upstream/main
git push upstream use-mapstr-from-libs

[kvch]

/test

[elasticmachine]

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