[KTextWidgets] Port from QRegExp to QRegularExpression

Summary:
Deprecate the KFind::find() and KReplace::replace() methods that take a
QRegExp; Regular Expression matches will be handled internally if the
KFind::RegularExpression flag is set.

Port QRegExp::setMinimal() by making the regex pattern non-greedy, where
possible.

Test Plan:

• make && ctest
• a quick test of searching and replacing in ktexteditest still works

Reviewers: Frameworks, mlaurent, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27097