Summary

Move the latest cell of the CSFLE mocked-KMS Linux test matrix 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+ — ubuntu2004-small no longer receives fresh latest server builds. Its stale build predates the substring feature flag (SERVER-128600) being enabled, so the QE "substring" GA tests added in CSHARP-6105 fail on the CSFLE Mocked KMS latest ubuntu-2004 variant with:

Enumeration value 'substring' for field 'create.encryptedFields.fields.queries.queryType' is not a valid value.

This is the minimal CI fix that unblocks verification of CSHARP-6105. Only latest is affected: it is the only CSFLE Linux variant that runs the substring GA tests (gated on server 9.0+) and that ubuntu-2004 can no longer build fresh. The 6.0/7.0/8.0 cells still have ubuntu-2004 builds and don't run substring tests; migrating them is consistency work tracked in CSHARP-6111.

Scope

Only the latest cell of the CSFLE mocked-KMS Linux matrix (csfle-with-mocked-kms-tests-linux-*) in evergreen/evergreen.yml.

Approach

• Add an ubuntu-2204 os-axis value: OS: "ubuntu-2204", run_on: ubuntu2204-small.
• Split the CSFLE mocked-KMS Linux matrix: keep 4.4, 5.0, 6.0, 7.0, 8.0, rapid on ubuntu-2004, and run latest on ubuntu-2204.

add-ca-certs.sh matches [[ "$OS" =~ Ubuntu|ubuntu ]], so ubuntu-2204 needs no script change.

Note: if rapid ever moves to a 9.0+ build it would need the same treatment (or it will be covered by CSHARP-6111).

Acceptance Criteria

• The CSFLE substring spec/prose tests pass on the latest Linux variant (now on Ubuntu 22.04).
• All other CSFLE Linux cells (4.4–rapid) still resolve on ubuntu-2004.
• An Evergreen patch across the CSFLE Linux variants is green.