Migrate/Discontinue greenhost machines

[hellais]

We should do the following in order to fit within the 50 credits/month tier:

To do by January 2025

• db-1.proteus.ooni.io (37.218.242.79) 56.80 -> see: Migrate db-1.proteus.ooni.io #128
• ams-matomo.ooni.nu (37.218.242.173) 34.10 -> see: Migrate ams-matomo.ooni.nu #129
• openvpn-server1.prod.ooni.io 14.80 -> Migrate over to digital ocean, migrate openvpn server to digital ocean #195

To do after

• ams-ps.ooni.nu (37.218.245.90) 31.30 -> we should either switch to using ooni-backend for this or keep it but switch to use prod postgres DB. This is needed for returning test targets to probes. discontinue ams-ps.ooni.nu #196
• [ ] mia-echoth.ooni.nu (37.218.241.93) 7.40 -> keep or migrate, yet migration requires writing code to support this legacy codebase
• [ ] mia-httpth.ooni.nu (37.218.241.94) 7.40 -> keep or migrate, yet migration requires writing code to support this legacy codebase
• [ ] ams-slack-1.ooni.org (37.218.247.98) 7.40 -> keep or migrate