Remove AppVeyor entirely (#18556)
Part of #17878
Blocked by release of 2025.2
Summary of the issue:
We are migrating from AppVeyor to GitHub actions.
After we migrate snapshots/PRs over, we will remove all AppVeyor related code.
Description of user facing changes:
None
Description of developer facing changes:
Remove all Appveyor related code. NVDA will only be built on GitHub actions.
Description of development approach:
Update documentation and remove code
