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`

(cherry picked from commit a277b56f6ae02d3dea8f20fb50a73a57c94e54ff)
Service-Card-Id: 85036583
Service-Version: 1.15