Temporarily explicitly default to EXCLUDE_DEPRECATED_BEFORE_AND_AT 5.99.0
Same as kwallet!47 (merged), the other left KF module which still has unremoved code blocks around.
Everything else no longer needs the special setup by developers adding the external flag (which actually could also have been avoided by properly setting the default value to 5.99.0 in all modules instead of the original 0, the external setting only made really sense for the special Qt6 builds of KF5 :) ).
One could actually argue that the new default should be 5.240 or something other pre-6.0.0, given that all deprecated API ideally is dropped before 6.0.0. But in that case 0 or e.g. 5.255.0 have the same meaning (everything before 6.0, which there is nothing actually), and 0 might be less specific and raising questions.