[MacPorts] #66975: Update qt6 to 6.4

[MacPorts]

#66975: Update qt6 to 6.4
-------------------------+--------------------
 Reporter:  MaddTheSane  |      Owner:  (none)
     Type:  update       |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:  2.8.1
 Keywords:               |       Port:  qt6
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 When attempting to build TerraFirma, I ran into a problem with cmake not
 knowing about `qt_standard_project_setup`. After some digging, it looks
 like it is provided by qt6, but qt6 6.2.4.

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Ticket URL: <https://trac.macports.org/ticket/66975>
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