Extensions - Change new tag "hidden" to "mgmt:hidden"

Extensions - Change new tag "hidden" to "mgmt:hidden"

Overview
--------

This is follow-up from the discussion about how to name extension tags

https://github.com/civicrm/civicrm-dev-docs/pull/761

It's a follow-up #16531 (dev/financial#84) within the same 5.24.alpha1 dev cycle.

Before
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The `sequentialcreditnotes` extension is tagged `hidden` and omitted
from display in the web UI.

After
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The `sequentialcreditnotes` extension is tagged `mgmt:hidden` and omitted
from display in the web UI.