Build with EXCLUDE_DEPRECATED_BEFORE_AND_AT=CURRENT

Summary:
To also support EXCLUDE_DEPRECATED_BEFORE_AND_AT=5.69.0 properly some
more if-elif would have been needed, making the code more hard to read.
Based on dicussion on #kde-devel with dfaure that has been left out,
for some future non-version-based *_BUILD_DEPRECATED.
So the focus already is on building without any own deprecated API,
no-one is expected to use EXCLUDE_DEPRECATED_BEFORE_AND_AT with a
certain version != CURREENT or 0.

Test Plan:
Builds with EXCLUDE_DEPRECATED_BEFORE_AND_AT set to 0 (default) or CURRENT,
all unit tests pass.

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D29572