[Bug] _disableContribution is not supported on modern template

**Describe the bug**
The modern template does not seem to allow setting `_disableContribution`, even though it is listed as supported in the documentation for the modern template: https://dotnet.github.io/docfx/docs/template.html?tabs=modern#template-metadata

**To Reproduce**
Steps to reproduce the behavior:
1. Specify metadata field in a specific article:
```md
---
_disableContribution: true
---
```
2. After building, the "Edit this page" button will remain active.

**Expected behavior**
The "Edit this page" button should not be displayed for the specific page.

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Context (please complete the following information):**
- Browser: chrome
- Form factor: desktop
- Docfx version: [e.g. 2.75.3]
- Template: modern