Cherry-pick #21159 to 7.x: [Filebeat] Add backwards compatibility for append allow_duplicates

[marc-gr]

Cherry-pick of PR #21159 to 7.x branch. Original message:

What does this PR do?

When loading a pipeline this change checks the elasticsearch version
and if the version is less than 7.10.0 it will replace the
"allow_duplicates: false" option with an equivalent if statement on the append
processor.

Why is it important?

This allows filebeat > 7.10.0 to be used with older versions of
elasticsearch. Without it the pipelines fail to load because the
option isn't supported.

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/siem (Team:SIEM)

[elasticmachine]

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Build stats

• Build Cause: [Pull request #21191 opened]

• Start Time: 2020-09-21T06:55:48.329+0000

• Duration: 58 min 18 sec

Test stats 🧪

Test	Results
Failed	0
Passed	5614
Skipped	833
Total	6447