PYTHON-3042 Migrate OCSP testing to Ubuntu 20.04 (mongodb#826)

Author: behackett

.evergreen/config.yml

@@ -2512,8 +2512,10 @@ buildvariants:
 
 - matrix_name: "ocsp-test"
  matrix_spec:
- platform: awslinux
- python-version: ["3.6", "3.9", "pypy3.6", "pypy3.7"]
+ # OCSP stapling is not supported on Ubuntu 18.04.
+ # See https://jira.mongodb.org/browse/SERVER-51364.
+ platform: ubuntu-20.04
+ python-version: ["3.6", "3.10", "pypy3.6", "pypy3.7"]
  mongodb-version: ["4.4", "5.0", "latest"]
  auth: "noauth"
  ssl: "ssl"