www/grafana8: Update to 8.1.0

Changelog:
https://grafana.com/docs/grafana/latest/whatsnew/whats-new-in-v8-1/

Also small port refactoring:

• Excluded GH_TUPLE to dedicated Makefile.modules file.
• Removed config sample file grafana.ini.in, refactored to auto-generation via Makefile (it prevents unnecessary file patching

while update).

PR: 257655
Reported by: maintainer