net-mgmt/prometheus2: Update to 2.26.1

Changes: https://github.com/prometheus/prometheus/releases/tag/v2.26.1

PR: 256324
MFH: 2021Q2 (security fix)
Security: CVE-2021-29622
Security: 59ab72fb-bccf-11eb-a38d-6805ca1caf5c
Submitted by: David O'Rourke <dor.bsd@xm0.uk> (maintainer)
Approved by: lwhsu (mentor)