www/grafana7: Deprecate

 * 7.x branch is deprecated upstream
 * Has unfixed vulnerabilities
 * grafana8 and grafana9 are available as replacements
 * no consumers of grafana7 in the ports tree

PR:		266535
Reported by:	drtr0jan@yandex.ru
Approved by:	robsonmantovani@gmail.com (maintainer, timeout > 2 weeks)