Disable AppVeyor for PRs/snapshots
Part of #17878
Summary of the issue:
As we transition from AppVeyor to GitHub Actions, we are nearly ready to deploy snapshots and releases via GHA.
For safety, we will perform tagged releases in the 2025.2 release cycle with AppVeyor still.
Description of user facing changes:
none
Description of developer facing changes:
AppVeyor will only build tagged releases.
PR builds, try builds and snapshots will now be built by GitHub actions.
Description of development approach:
removed lines from appveyor.yml. Appveyor uses master's copy of this file for all releases, unlike GitHub actions where it depends on the branches involved.
