Remove .NET Monitor 9.1 preview images

[lbussell]

.NET Monitor 9.1 preview has been superseded by .NET Monitor 10.0 and thus will not be released.

The images should be removed from the nightly branch now, and then removed from the main branch on the next Patch Tuesday (currently that's 2025-12-09).

• Remove Monitor 9.1 preview from nightly branch - Remove .NET Monitor 9.1 preview images #6822
• Remove Monitor 9.1 preview from main branch

/cc @wiktork