Upgrade WinUI/MUX to 2.7

[jonthysell]

This PR upgrades our MUX dependency from 2.6 to 2.7.

Closes #8724

Microsoft Reviewers: Open in CodeFlow

[asklar]

current 2.7 prerelease has a bug for xaml islands so we should hold off. See microsoft/microsoft-ui-xaml#5793

[jonthysell]

current 2.7 prerelease has a bug for xaml islands so we should hold off. See microsoft/microsoft-ui-xaml#5793

Is this fixed in the later 2.7.0-prerelease.210913003 (which is what I put in the PR)?

Or alternatively, does it repro in Microsoft.UI.Xaml.2.7.0-prerelease.210816001? which is what FB is using.

[asklar]

yes, 210816001 will reproduce the problem

[jonthysell]

Playground win32 builds just fine with 2.7.0-prerelease.210913003, which no longer depends on WebView2.

[NickGerleman]

We ought to start testing some community modules in our repo against canary builds. Like, a CI job that tries to build gallery against its latest bits.

[NickGerleman]

That was re things like having coverage we can compile react-native-webview.

[asklar]

woot :) 🎉
should this go into 67? if so we need to port it to the 0.67-stable branch

[jonthysell]

woot :) 🎉 should this go into 67? if so we need to port it to the 0.67-stable branch

I think so, along with #8723.