[MacPorts] #71013: qt6: Update to future 6.8.0 to support macOS 15.

[MacPorts]

#71013: qt6: Update to future 6.8.0 to support macOS 15.
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  Reporter:  cgilles  |      Owner:  MarcusCalhoun-Lopez
      Type:  update   |     Status:  assigned
  Priority:  Normal   |  Milestone:
 Component:  ports    |    Version:
Resolution:           |   Keywords:  sequoia
      Port:  qt6      |
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Comment (by reneeotten):

 I am preparing the update to 6.7.3 - it all builds except for
 {{{qt6-qtwebengine}}}; I don't want to switch to an unreleased version but
 will update once the final one is released.
 Again, if you want to help feel free to do so. The work-in-progress PR is
 [https://github.com/reneeotten/macports-ports/tree/qt6 here]. Everything
 builds on Sequoia except for {{{qt6-qtwebengine}}}. I haven't figured out
 yet how to resolve the issue, if you do please let me know.

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Ticket URL: <https://trac.macports.org/ticket/71013#comment:2>
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