Update to latest amazon linux image

[cat5inthecradle]

The Amazon Linux 2 3.0 image will go out of support in April, after which our pipeline may be slowed or eventually stop working.

• updates the CI image to "aws/codebuild/standard:5.0", which is not a required upgrade, just taking the opportunity to move to the Ubuntu 20 image.
• updates the CD image to the same "aws/codebuild/standard:5.0" from the Amazon Linux image. Switching to Ubuntu 20 from Amazon Linux because the newer Amazon Linux image does not support Java 11 without extra setup.

Note that all environments will need to have deploy-cicd.sh run after this is merged. See cicd/README.md

Testing

• creating a new development environment from this branch to confirm the new image works

Next Steps

• When we eventually update to Java 17, we could consider switching back to Amazon Linux, which may make builds faster.