Qt6: Update to 6.7.3

As a patch release, Qt 6.7.3 does not introduce new features but
contains more than 400 bug fixes, security updates, and other
improvements to the top of the Qt 6.7.2 release. See more information
about the most important changes and bug fixes from Qt 6.7.3 release
note.

PySide6: Update to 6.7.3

New port: devel/qt6-grpc

Qt GRPC is the client-side implementation that, together with the Qt
Protobuf module, allows the definition of messages and services in
.proto files, which then use the provided code generators to generate
client code that allows the access for fields and gRPC services in the
Qt framework. The code generated by Qt GRPC enables the client-side to
communicate with a gRPC server (regardless of whether the server uses
Qt) by sending calls or streaming messages.

Announcement: https://www.qt.io/blog/qt-6.7.3-released
Release note: https://code.qt.io/cgit/qt/qtreleasenotes.git/about/qt/6.7.3/release-note.md