chore: shut down test3

[Kouteki]

Description

Time to shut down the test3 testnet. It served us well.

Acceptance criteria

• Take down the test3 server
• Take down Grafana

[sw360cab]

Summoning @albttx as I don't have the required access to the used provider for that.

[leohhhn]

Please ping me when this is completed and I'll remove references from the docs.

[albttx]

@leohhhn this is done, you can remove test2 and test3

[leohhhn]

#3259