fix: skip validation for unpublish operations with localized required fields (#16120)

## Problem

Unpublishing documents with `required: true` + `localized: true` fields
fails with a validation error when `localization.fallback: false`.

The `UnpublishButton` sends only `{ _status: 'draft' }` with
`unpublishAllLocales=true` but no field data, causing `beforeChange`
validation to fail on missing required fields.

## Solution

Added `unpublishAllLocales` to the `skipValidation` condition in
`beforeChange` for both collections and globals — same pattern already
used for `isSavingDraft`.

## Why

Unpublish is a metadata-only operation (changing `_status`). Field
validation is unnecessary since data already exists in the database.

Fixes #15651

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Co-authored-by: Patrik Kozak <35232443+PatrikKozak@users.noreply.github.com>