[OOBE] Ensure the Settings button on the SCOOBE page opens Home, not a blank page (#46203)

## Summary of the Pull Request
This PR fixes an issue where selecting the **Settings** button on the
What's New page for a new or upgraded installation of PowerToys would
show the Settings application but with a blank contents page instead of
the Home page.

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## PR Checklist

- [x] Closes: #46202
<!-- - [ ] Closes: #yyy (add separate lines for additional resolved
issues) -->
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments
When the current version of PowerToys doesn't match the
`last_version_run`, the What's New (SCOOBE) page is displayed. The
Settings page is loaded at the same time in a hidden state.

If the user selects the Settings button in the bottom-left of the What's
New page, `OpenSettingsItem_Tapped()` is called, which calls:

```csharp
App.OpenSettingsWindow();
```

This unhides the Settings window, but Settings has not navigated to an
initial page, resulting in a blank display.

The solution is to instead call:

```csharp
App.OpenSettingsWindow(ensurePageIsSelected: true);
```

## Validation Steps Performed

Manual tests, following the instructions given in the original issue,
i.e. setting the `last_version_run` JSON manually and retrying to
simulate the upgrade/new install.