www/grafana6: Mark deprecated, for removal on 2022-03-31

Details:

• 6.x branch is deprecated upstream
• Has unfixed vulnerabilities
• grafana7 and grafana8 are available as replacements
• no consumers of grafana6 in the ports tree

PR: 261560
Reported by: Boris Korzun <drtr0jan@yandex.ru>
Approved by: Maintainer timeout (4 weeks)
MFH: 2022Q1
Security: CVE-2022-21702

		CVE-2022-21703
		CVE-2022-21713

(cherry picked from commit 091bf2a40acc9257a3bedbd5782eb3d7c6b714ef)