Does two things:

* the first two commits: shakes up the way we reference MUX in our projects so we can actually just 
  ```xml
  <PropertyGroup Label="NuGet Dependencies">
    <TerminalMUX>true</TerminalMUX>
  </PropertyGroup>
  ```
  Like every other dependency we have
* the last commit: update to MUX.2.7.3

This is the 1.14 PR, which should be appropriately cherry-picked through to `release-1.15` and `main`