Replace current issue templates with forms (and tweak guidance) #588

This is a rollup of two tasks:

1. We should replace the current issue templates with form templates, similar to what we did with gh-action-pip-audit;
2. We should enhance the guidance in the bug report form template to make report duplication less likely. In particular:
   1. Tell users to make sure they're running the latest stable release of pip-audit before making a report;
   2. Tell users to check recent open (and closed) bug issues before filing a new one.