images: Make 60 the default quality for AVIF

[bep]

AVIF's quality scale is not perceptually comparable to JPEG/WebP:
libavif anchors its "good" default at 60, where Hugo's universal 75
produced visibly higher quality and larger files. AVIF now defaults to
60 while JPEG and WebP keep 75. An explicit imaging.quality,
imaging.avif.quality or a per-image qNN still wins.

Fixes #14979

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com

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[jmooring]

Could we deprecate the imaging.quality config, and set defaults for imaging.avif.quality, imaging.jpeg.quality, and imaging.webp.quality instead? It would be easier to explain. Given what we now know/have, if we were starting from scratch we wouldn't have a global quality setting.

[bep]

Could we deprecate the imaging.quality config, and set defaults for imaging.avif.quality, imaging.jpeg.quality, and imaging.webp.quality instead?

Yea, we should do that.