Add proper warnings when attempting to use legacy QtWebKit

We need to clear up current situation of QtWebKit, properly mark deprecated versions (so called legacy, everything older than 5.212 branch) and let users know that there is important upgrade available.

There should be configure (and maybe even runtime) warning when attempting to use older versions (at least added to branches of LTS releases) which should also mention 5.212 as drop-in replacement, for example some link to "generic" wiki article about upgrading from legacy, preferably without mentioning any version in URL (same could apply to deprecation warnings).

This should help a bit with adoption of up to date version.