image/jpeg: replace fdct, idct with new code

[rsc]

CL 705518 replaced image/jpeg's DCT implementations with new ones that are more efficient, more accurate, more readable, and most importantly more Go-licensed. Users may notice slight differences in encoder/decoder output, so we should mention this in the release notes. Adding an issue here so I have a number for the release note file.

[gopherbot]

Change https://go.dev/cl/706736 mentions this issue: doc/next: document new image/jpeg DCT in release notes

[gabyhelp]

Related Code Changes

• image/jpeg: replace fdct.go and idct.go with new implementation in dct.go

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}