Support regenerating for existing versions

Current output generation is efficiently capped to ArduPilot tags that are newer than the latest commit in this repository.

That's nice to avoid unnecessary extra processing time, but it can also be useful to occasionally regenerate files for older versions, in case there have been changes in parameters that are reported as supported, and to include new MAVLink messages in the explicitly unsupported lists (e.g. as in #10).

It would be useful to add a command-line argument to the `run_parsers` script that allows overriding the "latest" date, and potentially running that via CI at a lower frequency (e.g. once a month).

Start with the run_parsers script and inspect how it determines the latest commit or date and handles command-line arguments. Define the override behavior for regenerating older ArduPilot versions, then verify that the normal cap remains unchanged; assess whether a lower-frequency CI invocation is also included.

Written by the indexing model from the issue text.