Improve Qt configuration in cmake: correct minimum version and avoid deprecation warnings

In this MR two small fixes were made regarding the Qt configuration with cmake:

• the minimum required version has been changed to 5.12 because it is needed by some code (e.g. in GUI/Models/RealDataItem serialisation via a QDataStream: version of data stream is expicitly set to 5.12)
• if a Qt version of at least 5.15 is found, the flag QT_NO_DEPRECATED_WARNINGS is set to suppress warnings. Background: to ease the migration to Qt 6 some functions were renamed with the new Qt 6 names. The old name of those functions is still available but marked as deprecated. Said flag will suppress those warnings, because in Qt 5.12 only the old function names are available and that version is the minimum supported by BornAgain.