cmake: Remove more Qt 5 code

[RytoEX]

Description

Qt Gui virtualkeyboard plugin was removed in Qt 6.x.
Qt Network Bearer Management was removed in Qt 6.0.
Qt Multimedia mediaservice and audio plugins were removed in Qt 6.x.

Motivation and Context

We don't support building with Qt 5 anymore, so we can remove Qt5-isms.

https://doc.qt.io/qt-5/bearer-management.html
qt/qtmultimedia@9814896
qt/qtmultimedia@762eff8
qt/qtmultimedia@b4d331d
https://doc.qt.io/qt-5/qtvirtualkeyboard-deployment-guide.html
https://doc.qt.io/qt-6/qtvirtualkeyboard-deployment-guide.html

How Has This Been Tested?

Tested locally on Windows 11.

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.