Switch to Github actions

[gadomski]

Description

1. Move the Gitlab merge request template to a Github pull request template
2. Remove deploy template and demo dev environment references from the pull request template. The deploy template is still private (and may end up being unused), and we currently do not have a process for updating the demo dev deployment from this repo. Ideally, we will use Github deployments to update the demo dev (or some environment) with changes, but that's an area for future work. I've opened Use Github deployments to update an enviornment on every PR merge #1 to track.
3. Move our continuous integration from gitlab runners to Github actions. This turned out to be pretty simple -- our CI was simply building and copying the stac-server lambdas. I've refactored the copying code out to a script, so it can be used locally as well as on CI. I've removed the dev/prod distintion, since it was unused in practice. If we need to separate dev and prod stac-server versions, we can re-add that in the future.
4. I sorted the .gitignore.