ORC-1854: Remove ubuntu20 from os-list.txt

### What changes were proposed in this pull request?

This PR aims to remove `ubuntu20` from `os-list.txt` at Apache ORC 2.1.1.

### Why are the changes needed?

When we remove `Ubuntu 20.04 Support` at ORC 2.1.0, we missed this.

- #1983

### How was this patch tested?

Manual review.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #2138 from dongjoon-hyun/ORC-1854.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: dongjoon-hyun

docker/os-list.txt

@@ -1,6 +1,5 @@
 debian11
 debian12
-ubuntu20
 ubuntu22
 ubuntu24
 fedora37