[MacPorts] #66975: qt6: Update to 6.4

[MacPorts]

#66975: qt6: Update to 6.4
--------------------------+--------------------
  Reporter:  MaddTheSane  |      Owner:  (none)
      Type:  update       |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  ports        |    Version:  2.8.1
Resolution:               |   Keywords:
      Port:  qt6          |
--------------------------+--------------------

Comment (by MaddTheSane):

 Well, now DuckStation wants 6.5.1, and looks like Sigil might want 6.5.2
 (ref #67863).

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Ticket URL: <https://trac.macports.org/ticket/66975#comment:4>
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