audio/strawberry: Update to 1.0.20

Version 1.0.20 is a hotfix release on top of 1.0.19.

Add EBUR128 option for loudness normalization analysis.

Switch default flavor to qt6. Upstream has recommended building against
Qt6 whenever possible for 18 months.

https://github.com/strawberrymusicplayer/strawberry/releases/tag/1.0.20
https://github.com/strawberrymusicplayer/strawberry/releases/tag/1.0.19