net-mgmt/prometheus2: Update to 2.26.0

This release changes the default AlertManager API to v2 and changes
promtool to output to stdout by default, in addition to other
enhancements and added features.

Changes https://github.com/prometheus/prometheus/releases/tag/v2.26.0

PR: 255021
Approved by: David O'Rourke <dor.bsd@xm0.uk> (maintainer)