proxmox-headers not pve-headers in PVE 9

Kernel headers package renamed from `pve-headers-$(uname -r)` to `proxmox-headers-$(uname -r)` in PVE 9. Old name is transitional.

Use proxmox-headers for DKMS builds. Discovered during NVIDIA GPU driver installation (2026-03-01).
Affected entities:NVIDIA GPU Driver