Cherry-pick #22521 to 7.x: Add basic wildcard/keyword fallback for upcoming ECS releases

[andrewstucki]

Cherry-pick of PR #22521 to 7.x branch. Original message:

What does this PR do?

This sets the groundwork for #21674 by adding a simple fallback mechanism for wildcard fields in unsupported Elasticsearch versions and non-OSS Beats distros. If you're publishing to ES < 7.9.0 you get keyword fields instead of wildcard, if you're coming from OSS Beats you get the same.

This will allow us to move forward with the migration to ECS 1.7.0 with upcoming wildcard support..

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/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

💚 Flaky test report

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Test stats 🧪

Test	Results
Failed	0
Passed	16376
Skipped	1368
Total	17744

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• Build Cause: [Pull request #22568 opened]

• Start Time: 2020-11-12T14:12:14.940+0000

• Duration: 64 min 55 sec

Test stats 🧪

Test	Results
Failed	0
Passed	16376
Skipped	1368
Total	17744

[andrewkroh]

👍