[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 jhoyt4):

 I had generated a PR for 6.5 here:[https://github.com/macports/macports-
 ports/pull/18186]

 While working on this, the port file and qt6 port group were significantly
 changed by the maintainer and updated to version 6.4.  Unfortunately, I
 ran out time to wrap my head around the new method as well as run the
 appropriate testing and had to close the PR.

 Someone else could certainly pick up where I left off.

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Ticket URL: <https://trac.macports.org/ticket/66975#comment:6>
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