Update defaultAmiId to 20-04 ubuntu image

⚠ ️NOTE THAT THIS FORK IS MAINTAINED FOR CRITICAL BUG FIXES AFFECTING RUNNING COSTS ONLY. NO OTHER CONTRIBUTIONS WILL BE ACCEPTED. ⚠️

What critical bug this MR is fixing?

Ran into this issue with the default Ubuntu 16.04 AMI whilst setting up a testing environment with AWS Autoscaling spot runners:

Installing Docker...                              [0;m  driver[0;m=amazonec2 name[0;m=runner-xxx-gitlab-runner-docker-machine-1651034439-c2a41596 operation[0;m=create
[31;1mERROR: Error creating machine: Error running provisioning: error installing docker: [0;m  [31;1mdriver[0;m=amazonec2 [31;1mname[0;m=runner-xxx-gitlab-runner-docker-machine-1651034439-c2a41596 [31;1moperation[0;m=create

I was unable to follow along with current docs regarding EC2 autoscaling to get this working with the default ami-c60b90d1, however with the AMI set to any newer version (18.04, 20.04) in config.toml the Installing Docker step completes without issue.

How does this change help reduce cost of usage? What scale of cost reduction is it?

Reduces likelihood of customers attempting to set up autoscaling following our Docs with the current default ami-c60b90d1 and running into these same issues.

In what scenarios is this change usable with GitLab Runner's docker+machine executor?

All scenarios, updates base defaultAmiId = "ami-08d4ac5b634553e16" :

ubuntu/images/hvm-ssd/ubuntu-focal-20.04-amd64-server-20220610
ami-08d4ac5b634553e16