Define standard CircleCI executors

[jdeanwallace]

Related https://github.com/tiny-pilot/tinypilot-pro/issues/1229

In an effort to reduce repetition on our CircleCI config, this PR defines standard CircleCI executors instead of redefining them on every job.

Notes:

1. FYI, CircleCI's cimg/base is an Ubuntu image:

   cimg/base is an Ubuntu Docker image created by CircleCI with continuous integration builds in mind. As its name suggests, this image is designed to serve as a base image for other CircleCI Convenience Images (images prefixed with cimg/).

2. This means that most jobs would use the same base image and are now being upgraded to the latest available image (i.e., cimg/base:2024.02)
   • We can't upgrade the base images on all jobs because some rely on very specific versions. For example: https://github.com/tiny-pilot/tinypilot/blob/399637566b5b54d5c881dcf2cb43dfaa560ff236/.circleci/config.yml#L171-L173

[jdeanwallace]

Automated comment from CodeApprove ➜

⏳ @jotaen4tinypilot please review this Pull Request

[mtlynch]

Oh, neat! I didn't know about this. I need to go refactor like 50 repos now.

This means that most jobs would use the same base image and are now being upgraded to the latest available image (i.e., cimg/base:2024.02) We can't upgrade the base images on all jobs because some rely on very specific versions.

Does that job actually rely on that specific image or did we just give it an overly specific tag back when we created it? If it specifically depends on that exact tag, can we add a comment in this PR explaining why?

[jdeanwallace]

@mtlynch

I need to go refactor like 50 repos now.

My pleasure! 😁

Does that job actually rely on that specific image...

Good question 🤔 I think we made it that specific ubuntu image version back in the day because we wanted at least lintian=2.114.0ubuntu1, but now the new ubuntu image can install newer lintian versions (specifically 2.114.0ubuntu1.3) anyway. So yay, we can upgrade all base images 🙌