PSA: Qt6-based KDE software builds: drop EXCLUDE_DEPRECATED_BEFORE_AND_AT override now

Hi,

a quick heads-up:

if you already package or otherwise do any Qt6-based KDE software builds and 
have in the set of manual flags passed to cmake any
    "-DEXCLUDE_DEPRECATED_BEFORE_AND_AT=x.y.z"
override, please remove that override completely now.

All KDE sources should now set the proper default value themselves (which also 
was "5.99.0" only for Frameworks modules, other libraries would have needed 
other values matching their respective versioning scheme, things worked only 
by chance).

EXCLUDE_DEPRECATED_BEFORE_AND_AT is a flag which controls what API 
implementation parts of a library should be excluded when building it.
So indeed is not about excluding usage of deprecated API from other libraries.
BUILD_DEPRECATED_AFTER_AND_AT might have been a better name, perhaps something 
to switch to :)

See also
https://invent.kde.org/sdk/kdesrc-build/-/merge_requests/248
https://invent.kde.org/sdk/kdesrc-build/-/merge_requests/250
https://invent.kde.org/sysadmin/ci-utilities/-/merge_requests/106

If you know any place which still holds that flag e.g. in instructions, please 
also update or report.

Cheers
Friedrich