package/qt5: drop 5.6 support

As discussed during the FOSDEM2019 develop days, Qt 5.6 is very old (5.6.3
was released in September 2017, and 5.6.x became EOL in March 2019), so drop
it before the new Buildroot LTS release:

https://elinux.org/Buildroot:DeveloperDaysFOSDEM2019#Qt5_versions_to_support:_keep_5.6_or_a_newer_LTS.3F



And add legacy handling for it.

There are a number of places where code checks for
BR2_PACKAGE_QT5_VERSION_LATEST, so leave that as a blind option for now to
not break the build.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>