Cherry-pick #19586 to 7.x: Make Agent beta and Constraints experimental

[michalpristas]

Cherry-pick of PR #19586 to 7.x branch. Original message:

What does this PR do?

As for 7.9 Agent is in Beta and Constraints stays in experimental due to planned changes.
This PR marks this accordingly

Why is it important?

To avoid confusion in use cases and prevent relying on constraints

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]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[elasticmachine]

💚 Build Succeeded

Expand to view the summary

Build stats

• Build Cause: [Started by user Michal Pristas, Replayed #1]

• Start Time: 2020-07-03T12:06:53.986+0000

• Duration: 34 min 0 sec

Test stats 🧪

Test	Results
Failed	0
Passed	547
Skipped	128
Total	675