Add `concurrency` option for Github actions

[elliette]

Along with on, env, etc, Github actions now has the keyword concurrency which allows you to specify concurrency at the workflow level. This lets you cancel any previous workflows if there is a newer one.

It would be great if mono_repo would support concurrency. If you try to add it to your mono_repo.yaml, you see:

Unrecognized keys: [concurrency]; supported keys: [env, on, on_completion, dependabot, cron, stages, workflows]

[kevmoo]

PRs welcome!