[7.x](backport #27774) [Filebeat] Update compatibility function to remove processor description on ES < 7.9.0

[mergify]

This is an automatic backport of pull request #27774 done by Mergify.
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	modified:   CHANGELOG.next.asciidoc

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	both modified:   filebeat/fileset/compatibility.go
	both modified:   filebeat/fileset/compatibility_test.go

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[elasticmachine]

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• Start Time: 2021-09-09T20:48:39.498+0000

• Duration: 98 min 3 sec

• Commit: a7313cd

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Passed	15254
Skipped	2327
Total	17581

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Passed	15254
Skipped	2327
Total	17581

[legoguy1000]

@andrewkroh Looks like there are conflicts on the backport

[andrewkroh]

There was a backport missing #27841. After that merges I'll rebase this one.

[elasticmachine]

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

[botelastic]

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