Clarify Bandit Python version guidance

[Himanshuagrawal4]

Summary

• clarify when users need to run Bandit under different Python interpreters
• explain that the key compatibility boundary is whether the interpreter can parse the scanned source
• note that a current supported Bandit release is usually enough for projects whose source parses on one supported interpreter

Closes #1375

Verification

• git diff --check
• parsed doc/source/faq.rst with docutils.core.publish_doctree