Grafana version is inconsistent between docker images (and they are outdated)

[Elanis]

Hi,

I noticed that Grafana versions are outdated and not the same between images:

• using 6.7.6 in nonroot and default image
• using 8.3.4 in postgres image
• using 8.5.15 in timescale image

Is there a reason behind this choice ? Can we try to update these version ?

NB: For future usage, upgrade guides:

• v7: https://grafana.com/docs/grafana/v7.0/installation/upgrading/#upgrading-to-v7-0
• v8: https://grafana.com/docs/grafana/latest/setup-grafana/upgrade-grafana/#upgrading-to-v80
• v9: https://grafana.com/docs/grafana/latest/setup-grafana/upgrade-grafana/#upgrading-to-90

[Elanis]

Just saw in #544 that Influx versions are deprecated.
We should change documentation and README to stop using them, if this is going to disappear.