fix(terraform): Downgrade provider Proxmox vers v2.9 stable

The v3.0 provider is not yet available as a stable release in the
OpenTofu/Terraform registry. Downgraded to v2.9 which is the latest
stable version.

Also fixed minor yamllint issues in flux.yml:
- Added space after comment marker
- Removed trailing blank line

terraform/pve3/main.tf

@@ -4,7 +4,7 @@ terraform {
   required_providers {
     proxmox = {
       source  = "telmate/proxmox"
-      version = "~> 3.0"
+      version = "~> 2.9"
     }
     local = {
       source  = "hashicorp/local"