Promote experimental Window sizing APIs to stable

## What

Two new windowing APIs just landed in `winui3/main` as **experimental**. This issue tracks getting them from experimental all the way to **stable**.

### The APIs

**1. Window.Width / Window.Height** - set window size in device-independent pixels, straight from XAML. No more digging through AppWindow or juggling scale factors.
- Proposal: #2731
- Came in on commit dd6808e (ADO PR 15799583, from community PR #11052 by @dotMorten)
- API spec: #11171

**2. Window.MinWidth / MinHeight / MaxWidth / MaxHeight** - constrain how big or small a user can resize the window.
- Proposal: #7296
- Came in on commit 9a77fa1 (ADO PR 16117156)

## Work to get to stable

_Checklist restructured to track each API separately, with help from JeCollin's AI assistant. AI makes mistakes._

### Window.Width / Window.Height (proposal #2731, spec #11171)
- [ ] Finalize the API spec (#11171)
- [ ] Confirm test coverage
- [ ] Bake in an experimental release, gather feedback
- [ ] Flip from experimental to the stable contract
- [ ] Docs

### Window.MinWidth / MinHeight / MaxWidth / MaxHeight (proposal #7296)
- [ ] Finalize the API spec
- [ ] Confirm test coverage
- [ ] Bake in an experimental release, gather feedback
- [ ] Flip from experimental to the stable contract
- [ ] Docs

## Related proposals
- #2731 - Window Width/Height in Desktop
- #7296 - Adding Min / Max Window Size