This introduces a more automatic way of increasing the log levels for deprecation log statements based on the version it was deprecated.
The thresholds are a little arbitrary, but
* We log INFO for 6 releases
* We log WARN for another 6 releases
* THen ERROR (failing the build)
This should give theme authors plenty of time to catch up without having the log filled with warnings.
Primary motivation is documentation, but it will also hopefully simplify the code.
Also,
* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.
Closes#10896Closes#10620