fix(metal): upgrade Rocky Linux to 8.6 and pin permanent URL

Author: locmai

metal/roles/pxe-server/defaults/main.yml

@@ -1,3 +1,3 @@
-iso_url: "https://download.rockylinux.org/pub/rocky/8/isos/x86_64/Rocky-8.5-x86_64-minimal.iso"
-iso_checksum: "sha256:4eb2ae6b06876205f2209e4504110fe4115b37540c21ecfbbc0ebc11084cb779"
+iso_url: "https://download.rockylinux.org/pub/rocky/8.6/isos/x86_64/Rocky-8.6-x86_64-minimal.iso"
+iso_checksum: "sha256:a9ece0e810275e881abfd66bb0e59ac05d567a5ec0bc2f108b9a3e90bef5bf94"
 timezone: Asia/Ho_Chi_Minh