CDRIVER-4739 Migrate MongoDB 7.0+ Evergreen tasks from Ubuntu 18.04 to 20.04

[kevinAlbs]

Summary

Migrate MongoDB 7.0+ Evergreen tasks from Ubuntu 18.04 to 20.04

Tasks matching .*7.0.* were run in this patch build to verify: https://spruce.mongodb.com/version/651d7291c9ec44a912961b62. The failing tasks are expected and fail on the master branch.

Background & Motivation

MongoDB 7.0 does not officially support Ubuntu 18.04. Ubuntu 18.04 binaries were removed in 7.0.2 in SERVER-77233. This has resulted task failures for server 7.0 tasks on Ubuntu 18.04.

This PR intends to make the minimal change to reduce test failures by upgrading selected variants to Ubuntu 20.04.

[eramongodb]

Confirming, why was Ubuntu 20.04 chosen instead of Ubuntu 22.04?

[kevinAlbs]

Confirming, why was Ubuntu 20.04 chosen instead of Ubuntu 22.04?

A patch using Ubuntu 22.04 resulted in task failures using clang:

CMake Error at /home/ubuntu/.cache/mongo-c-driver/tmp.npvLOnLZ2D/cmake-3.25.3-linux-x86_64/share/cmake-3.25/Modules/CMakeDetermineCCompiler.cmake:49 (message):
  Could not find compiler set in environment variable CC:
  clang.

I did not investigate further. I support a future move to Ubuntu 22.04. For this PR, I propose making the minimal change to get the 7.0 tasks passing.