Update all .j2 file extension usages to .jinja

Since the templating engine changed names from Jinja2 to Jinja (see #197 for the preliminary set of changes I made), I would like to pick some point to rename all `.j2` template files to `.jinja`. Since this change is a little more far-reaching than just changing text references from Jinja2 to Jinja, I'm going to put this off until there's a bigger batch of breaking changes, e.g. after a larger Ansible release or when adding a bunch of content for Collections—something along those lines.