[9.0] TBS: change default disk usage threshold to 0.8 (backport #15524)

[mergify]

Motivation/summary

Change TBS default usage threshold to 0.8 for a lower risk. Change log to be slightly more readable.

With this change, the log looks like

setting disk usage threshold to 80% of total disk space of 912.7gb 

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

Check logs setting disk usage threshold ... when storage_limit is set to 0.

Related issues

Changing the default set in #15467

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This is an automatic backport of pull request #15524 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 9ccfae6 has failed:

On branch mergify/bp/9.0/pr-15524 Your branch is up to date with 'origin/9.0'. You are currently cherry-picking commit 9ccfae64. (fix conflicts and run "git cherry-pick --continue") (use "git cherry-pick --skip" to skip this patch) (use "git cherry-pick --abort" to cancel the cherry-pick operation) Changes to be committed:	modified: changelogs/all-breaking-changes.asciidoc	modified: internal/beater/config/config_test.go	modified: internal/beater/config/sampling.go	modified: x-pack/apm-server/sampling/eventstorage/storage_manager.go Unmerged paths: (use "git add/rm <file>..." as appropriate to mark resolution)	deleted by us: changelogs/9.0.asciidoc 

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally