[MacPorts] #58245: mpv: update config-maintainer to avoid deprecation warnings

[MacPorts]

#58245: mpv: update config-maintainer to avoid deprecation warnings
-------------------------+----------------------
 Reporter:  dbevans      |      Owner:  Ionic
     Type:  enhancement  |     Status:  assigned
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:
 Keywords:               |       Port:  mpv
-------------------------+----------------------
 When configuring using config-maintainer, mpv issues deprecation warnings
 as follows:

 {{{
 $ mpv alexander_nevski.mpeg
 Driver 'opengl' has been replaced with 'gpu'!
 Warning: option --opengl-backend was replaced with --gpu-context and might
 be removed in the future.
 Driver 'opengl' has been replaced with 'gpu'!
 Playing: alexander_nevski.mpeg
  (+) Video --vid=1 (mpeg1video 720x544 23.976fps)
  (+) Audio --aid=1 (mp2 2ch 48000Hz)
 [vo/opengl] opengl cocoa backend is deprecated, use vo=libmpv instead
 AO: [coreaudio] 48000Hz stereo 2ch s16
 VO: [gpu] 720x544 yuv420p
 AV: 00:00:16 / 00:30:11 (0%) A-V:  0.000 Cache: 1192s+199MB
 }}}

-- 
Ticket URL: <https://trac.macports.org/ticket/58245>
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