Ubuntu 20.04 (focal) reached end of standard support in May 2025, so this removes it from our CI testing on master.

Removes the five Ubuntu 20.04 build variants from the nightly project (`etc/evergreen_yml_components/variants/ubuntu/test_release.yml`):

• `ubuntu2004`
• `ubuntu2004-arm64`
• `enterprise-ubuntu2004-64`
• `enterprise-ubuntu2004-64-security`
• `enterprise-ubuntu2004-arm64`

The security team's OIDC variant (`enterprise-ubuntu2004-64-security`) is not dropped but migrated to Ubuntu 24.04 as `enterprise-ubuntu2404-security`: it reuses the new `enterprise-ubuntu2404` template via a YAML anchor and runs the same `external_auth_oidc`, `external_auth_oidc_azure`, and `external_auth_oidc_gcp` tasks against the `enterprise-ubuntu2404` build.

Also removes the references to the deleted variants from supporting config:

• `etc/evergreen_yml_components/definitions.yml` — package variants list
• `etc/evergreen_yml_components/tasks/compile_tasks_nightly.yml` — mongocryptd variants list
• `etc/evergreen_timeouts.yml` — `aggregation_blockprocessing_fuzzer` timeout overrides
• `etc/generate_subtasks_config.yml` — large-distro exception list
• `evergreen/package.sh` — valid mongocryptd variants list
• `buildscripts/resmokeconfig/setup_multiversion/setup_multiversion_config.yml` — multiversion buildvariant mappings

Intentionally left alone: `buildscripts/packager*.py`, `buildscripts/package_test.py`, and `etc/repo_config.yaml` still know about the `ubuntu2004` distro, matching the existing precedent for ubuntu1804 (these serve release branches that still ship focal packages). The Bazel ubuntu20 toolchain/RBE container definitions are also untouched since they are not tied to these test variants.

This only affects the master nightly project; release branch projects (vX.Y) have their own copies of these configs.