arrow

GH-44253: [CI][Release][Python] Do not verify Python on Ubuntu 20.04 (#44254)

### Rationale for this change
Python 3.8 is no longer supported for Pyarrow. Ubuntu 20.04 ships with Python 3.8

### What changes are included in this PR?

Remove verification checks on Ubuntu 20.04

### Are these changes tested?

Via archery

### Are there any user-facing changes?

No
* GitHub Issue: #44253

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>