Remove global.net11.json - not needed

The main global.json already targets .NET 11 SDK. The global.netX.json
files are only used by CI (ci_setup.py) to downgrade the SDK when
testing older .NET versions. A global.net11.json would only be needed
in the future when global.json moves to .NET 12.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: danmoseley