[grafana] feat: Defaults image.tag to chart appVersion

[KENNYSOFT]

There are many cases that PR or chart release only changed appVersion and left image.tag value (may not be the whole):

• chart 6.26.6 [grafana] bump chart to latest grafana version #1289
  • fixed in chart 6.26.7 [grafana] fix grafana version in values.yaml #1294
• (by me) chart 6.21.7 [grafana] deps: Bump grafana from 8.3.5 to 8.3.6 #1051
  • fixed in chart 6.21.8 [grafana] deps: Bump grafana from 8.3.5 to 8.3.6, take 2 #1052
• chart 6.20.3 [grafana] Only specify replicas if given a value #907
  • fixed in chart 6.20.6 [grafana] fix: Sync image tag with appVersion #975
• chart 6.16.7 [grafana] Update grafana chart for grafana 8.1.4 #692, chart 6.16.8 [grafana] Update chart with grafana 8.1.5 #694
  • fixed in chart 6.16.9 [grafana] Fix the image tag in the grafana chart #698

It seems first-comers can make mistakes easily, so I've changed the tag used by pod image definition defaults to chart appVersion.

Description based on: https://github.com/argoproj/argo-helm/blob/1492575ee07aebdbdc22ff4d16da7455fe3e1b1e/charts/argo-cd/values.yaml#L16