net-mgmt/prometheus2: Update to 2.28.1 (bugfix release)

• pet portclippy: reorder variables
• pet portfmt: reformat Makefile

ChangeLog: https://github.com/prometheus/prometheus/blob/main/CHANGELOG.md

PR: 257099
(cherry picked from commit 2691a5b56f006c479fe9cabaf5908538b665473d)