Change type of dashboard variables from constant to custom

[RincewindsHat]

With Grafana 11.3 most of the dashboards broke, due to Grafana enforcing that constants are
actually constant ( o.O ) (ref: grafana/grafana#90635 ).

Since this was a breaking change but actually a bugfix, the dashboards in here should be
updated.

This PR changes the type of all dashboard variabels from constant to custom.
It should not change anything really on Grafana < 11.3 and in general just restore the
functionality on >= 11.3

[RincewindsHat]

Reference:

• Grafana: URLs no longer set constant variables grafana/grafana#90635
• After upgrading to Grafana 11.3.0 passing variables as CONSTANTS does not work Mikesch-mp/icingaweb2-module-grafana#344
• URL variables no longer passed to the query grafana/grafana#97977

[RincewindsHat]

Basically same as: https://github.com/Mikesch-mp/icingaweb2-module-grafana/pull/346/files

[martialblog]

And #18

[martialblog]

I trust you have tested this.

[RincewindsHat]

well, I did not until now and the import fails. Sorry

[martialblog]

I deducted 1 from your trust pool

[RincewindsHat]

that's only fair

[martialblog]

I just some testing with Grafana 12 and I had to change the variable type as well #70

[martialblog]

@RincewindsHat I think we can merge this

[RincewindsHat]

ok