fix: deprecate unity-launch flag

[rzhao271]

Fixes #202545

This PR removes mentions of the unity-launch flag.
I have tested the deb and rpm with a custom dev build and found no issues when opening from the launcher with left click or ctrl+left click.

• Test opening from the dock with ctrl+left click versus opening a terminal from the dock with ctrl+left click

  • Ubuntu deb—both terminal and Insiders open a new instance
  • Ubuntu snap—both terminal and Insiders open a new instance
  • Fedora rpm—both terminal and Insiders open a new instance

• Test opening from the launcher with ctrl+left click versus opening a terminal from the launcher with ctrl+left click

  • Ubuntu deb—both terminal and Insiders open a new instance
  • Ubuntu snap—both terminal and Insiders open a new instance
  • Fedora rpm—both terminal and Insiders open a new instance in the background

• Test opening from the dock with shift+left click versus opening a terminal from the dock with shift+left click

  • Ubuntu deb—both terminal and Insiders open a new instance
  • Ubuntu snap—both terminal and Insiders open a new instance
  • Fedora rpm—both terminal and Insiders go to existing instance

• Test opening from the launcher with shift+left click versus opening a terminal from the launcher with shift+left click

  • Ubuntu deb—both terminal and Insiders go to existing instance
  • Ubuntu snap—both terminal and Insiders go to existing instance
  • Fedora rpm—both terminal and Insiders go to existing instance

[rzhao271]

I have confirmed that Insiders seems to behave the same as the terminal in all instances.

[Tyriar]

If --unity-launch happens to be invoked do we refuse to launch now? This could happen if the user forks the code.desktop to customize it.

[rzhao271]

@Tyriar I just confirmed that Code-Insiders still launches and warns that the option is not in the list of known options, but is still passed to Electron/Chromium.