refactor: migrate from ExAws to aws-elixir

[thecristen]

Not sure if this is a better choice. Some things I liked about aws-elixir:

• It's got types, super helpful for understanding the particulars of each function's inputs and outputs (I had to dig into the generated source code, but maybe still better than wading through AWS docs)
• Results come out already parsed, so no more reading responses / decoding JSON ourselves
• It's got literally everything (also a slight con - it takes a noticeably longer time to compile) - no crafting bespoke configurations for more generic functions

And I'll have to deploy this and poke around to ensure I didn't break things. We don't use AWS a ton, but the few things we do with it are fairly important.

This affects:

• searching for locations (AWS Location Service)
• saving and surfacing bus stop change alerts (AWS S3)
• mailing tickets to IRIS/HEAT (AWS Simple Email Service)

[anthonyshull]

Is this supposed to be here?

[thecristen]

Oh right, missed commenting on this. We could either write it here or require devs and CI to have this value saved to this environment variable, it's needed for tests to pass. It's the publicly available one for testing listed here: https://developers.google.com/recaptcha/docs/faq#id-like-to-run-automated-tests-with-recaptcha.-what-should-i-do

[anthonyshull]

If it's public then we can just keep it as a default.