Summary

Migrate the driver's Linux test build variants that run MongoDB server rapid/latest from Ubuntu 20.04 (ubuntu-2004 / ubuntu2004-small) to Ubuntu 22.04 (ubuntu-2204 / ubuntu2204-small).

Motivation

Ubuntu 20.04 is being EOL'd as a MongoDB server build target for 9.0+ — the ubuntu2004-small distro no longer receives fresh latest server builds. This first surfaced in CSHARP-6105 (QE "substring" GA): the CSFLE Mocked KMS latest ubuntu-2004 variant failed with Enumeration value 'substring' for field 'create.encryptedFields.fields.queries.queryType' is not a valid value, because its stale server build predates the substring feature flag (SERVER-128600) being enabled. Any ubuntu-2004 variant that runs latest (9.0+) is exposed to the same class of failure.

Scope

~20 build-variant matrices in evergreen/evergreen.yml currently pin os: "ubuntu-2004", many spanning the full server-version axis through latest. All of them need the newer distro for their 6.0+ cells.

Approach

• Add an ubuntu-2204 os-axis value: OS: "ubuntu-2204", run_on: ubuntu2204-small.
• For each Linux matrix, split the version axis:
  • 4.4, 5.0,6.0, 7.0, 8.0 remain on ubuntu-2004 (Ubuntu 22.04 has no 4.4/5.0 server builds).
  • rapid, latest move to ubuntu-2204 (Ubuntu 22.04 supports server 6.0+).
• Audit shell scripts that branch on $OS. add-ca-certs.sh matches [[ "$OS" =~ Ubuntu|ubuntu ]], so ubuntu-2204 is handled without change; confirm no other script hardcodes ubuntu-2004.

Acceptance Criteria

• All Linux test variants running server rapid/latest run on ubuntu2204-small.
• 4.4/5.0/6.0/7.0/8.0 Linux cells still resolve (stay on ubuntu-2004).
• An Evergreen patch across the affected variants is green.

Alternative considered (deferred)

Standardizing all Linux tests on a single RHEL 8 distro (MongoDB ships rhel80 server binaries across the full version range, which would remove the split-matrix entirely). Deferred because: (1) it ripples into $OS-branching shell scripts (e.g. add-ca-certs.sh's ubuntu-only regex), (2) a general rhel8* distro's availability in the dot-net-driver project is unconfirmed, and (3) it changes the environment for all Linux tests (larger blast radius). Can be revisited as a follow-up if the split-matrix maintenance proves burdensome.