ci: upgrade `sparse` to supported build agents

ci: upgrade `sparse` to supported build agents

The `sparse` job still uses the `ubuntu-20.04` runner pool, but that
pool is about to go away, so let's stop using it.

There is no `sparse-22.04` artifact provided by the "Build sparse for
Ubuntu" Azure Pipeline, but that is not necessary anyway because Ubuntu
22.04 has the `sparse` package: https://packages.ubuntu.com/jammy/sparse

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>