CI workflow to automatically bump the version of Prefect in prefect-operator

[chrisguidry]

In #35, we talked through the best approach to keeping up with new Prefect versions. The design we landed on is:

• Keep a hard-coded version number in the source code, isolated in a separate file
• React when Prefect releases are created (via a GH webhook call?) to run a CI workflow that bumps that hard-coded version
• That CI workflow will run the unit test suite, and if tests are passing and no other changes are queued up, cut a release