FC: update Firecracker to v0.20.0

The new release for Firecracker is `v0.20.0`.

Fixes: kata-containers#2378

Signed-off-by: Penny Zheng <penny.zheng@arm.com>

versions.yaml

@@ -83,7 +83,7 @@ assets:
       uscan-url: >-
         https://github.com/firecracker-microvm/firecracker/tags
         .*/v?(\d\S+)\.tar\.gz
-      version: "v0.19.1"
+      version: "v0.20.0"
 
     qemu:
       description: "VMM that uses KVM"