Minor fixes for NiFi 12.x matching release

• Bump version: 0.16.2 → 0.16.3
• Removed force reset of configuration.password and configuration.username to empty string. This was not increasing security, and was causing unexpected errors for users connecting to multiple services in a single script.
• Add greedy control to versioning.get_registry_bucket and versioning.get_flow_in_bucket to avoid undesirable partial string match.
• Update readme to reflect switch from 'master' branch naming to 'main'.
• Update tox to pin testing to Python 3.8, as Python 3.9 is producing unexpected and unrelated SSL failures
• Minor lint formatting improvements
• Update docs for 0.16.3 patch release
• Correct wording in Parameter Contexts warning when version support is unknown. It may drop the details, but it might also keep them.