Compute baseline_low_date from version numbers

[ddbeck]

For a few features, we can't rely on data from BCD because BCD doesn't represent that kind of support information (for example, codecs). In those cases, we set suport values manually. In such cases, it should be possible to record the support values and compute the baseline and baseline_low_date automatically, leaving those as optional overrides.

Inspired by #2436.