August 2021 week 33

- update macOS 11 box to 11.5.2
- update Windows 10 box, latest security fixes applied
  (but still Windows 10 20H2)
- first version of box for Windows 11, based on beta
- remove windows7 machine from default minicoin.yml,
  add windows11
- minor fixes to base box maintenance scripts
- install gstreamer requirements for linux-builder
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