MongoDB 7.0 does not officially support Ubuntu 18.04. Ubuntu 18.04 binaries were removed in 7.0.2. This has resulted in our test suite failing on Linux now that 7.0.2 is available.

In CSHARP-4624 we previously attempted to upgrade to Ubuntu 20.04, but ran into a variety of problems. We should either work around these issues or confirm that they're no longe present.

As well .NET Core 2.X - which we use for netstandard2.0 testing - is not available for Ubuntu 20.04 and newer.

Given that we don't ship server binaries for Ubuntu 20.04 for MongoDB 3.6, 4.0, or 4.2, I propose the following:

MongoDB 3.6 to 6.0 test against Ubuntu 18.04 running all TFMs.
MongoDB 7.0, rapid, and latest test against Ubuntu 20.04 running netstandard2.1 using netcoreapp3.1. We can also smoke test against more recent .NET versions such as .NET 5 and 6.

We may want to consider upgrading MongoDB 4.4+ to use Ubuntu 20.04 and only running MongoDB 3.6, 4.0, and 4.2 on Ubuntu 18.04. This would reduce our Linux test coverage as we wouldn't test netstandard2.0 on Linux as much as previously, but that may be acceptable as we would like to minimize how much we leverage EOL'd operating systems.

ASIDE: Initially I thought we could switch to RHEL7, but it EOLs June 30, 2024. Staying on Ubuntu seems like the easiest path forward.