Cherry-pick #16476 to 7.6: match reference.yml to code default

[urso]

Cherry-pick of PR #16476 to 7.6 branch. Original message:

What does this PR do?
Changes the default for setup.ilm.check_exists from false to true to match the code defaults. Closes issue #16474

The code defaults are in https://github.com/elastic/beats/blob/7.6/libbeat/_meta/config.reference.yml.tmpl#L978-L981

Why is it important?

Users depend on .reference.yml to be correct as they use it as a template for their own config files.

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

(the code defaults and docs are already correct, just the reference.yml files are wrong). The code checks are already there.

Author's Checklist

• Someone should verify that the template file I modified is used to generate all of the .reference.yml files, I do not know how to do this.

How to test this PR locally

Related issues

• Closes reference.yml files have incorrect default for setup.ilm.check_exists #16474