Upgrade RNX to target RNW 0.77

[jonthysell]

This PR bumps RNX to target RNW 0.77, with a minimum of RNW 0.72.

This PR also resolves such issues that have arisen:

• Renamed XAML's TabIndex property mapping from tabIndex (which conflicts with upstream) to xamlTabIndex
• Fixed yarn workspaces
• Fixed example app metro configs so they can be run
• Updated and aligned versions of packages in yarn.lock
• Updated workflows with latest GitHub Actions versions for common tasks
• Updated workflows to only target support RNW versions for new app testing
• Updated workflow nuget restores to use lock files
• Updated new app testing to use the new react-native init-windows

Resolves #287
Resolves #288

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