fix: add rsync protect filter to preserve openapi spec

[kruskall]

Motivation/summary

we're syncing docs spec from apm-data which causes rsync to delete the openapi spec because it's not in the target dir (apm-data). This PR adds a protect filter to prevent the openapi folder from being deleted.

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

run make docs/spec
openapi spec should still be there

Related issues

Closes #12904

[mergify]

This pull request does not have a backport label. Could you fix it @kruskall? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.

NOTE: backport-skip has been added to this pull request.

[endorama]

question: why this change? Does not seem related to the other change or with the linked issue to me, but I may lack the context. Could you clarify it?

[kruskall]

the folder doesn't exist anymore and the task is unused. The doc was moved in e0c42a5