Disable VSCode's built-in auto update by default

## Why

- Sometimes the Flatpak version is out of sync with the Microsoft release schedule
- The built-in update doesn't really have the ability to update the application due to the read-only essence of flatpak
- This practice is also found in applications like Firefox.

## How

### Approach 1

Unset `updateUrl` in `product.json` as what VSCodium does: https://github.com/VSCodium/vscodium/blob/9c8d4b367dadb4a4b007285a737233b8ee3cdf86/prepare_vscode.sh#L104-L106

### Approach 2

Set `UpdateMode` [policy](https://github.com/microsoft/vscode/blob/38b3b3179ecde98238c118343660388278c30378/src/vs/platform/update/common/update.config.contribution.ts#L32) to `none` in [Policy settings](https://code.visualstudio.com/docs/getstarted/settings#_settings-precedence). But I haven't figured out how to set the policy.