Update registration disappears during upgrade.

### Task

[Update registration](https://docs.firegiant.com/wix/schema/wxs/optionalupdateregistration/) is stored in a shared registry key that Burn takes care to keep around across upgrades. The approach it used broke between WiX v3 and WiX v5. [The PR](https://github.com/wixtoolset/wix/pull/622) makes it work again by not rewriting update registration during upgrade uninstall.