docs: migration headers keep the before and after, without ids, quotes or one library's numbers (#6277)

Applies rule 32's migration exception: each header still says what the
schema was, what changes and what it prevents, but task, milestone,
decision and rule ids are replaced by the practice they stood for, the
operator quotes by the decision, single-install counts are dropped, and
pointers to a deleted package and an external design doc are gone.
golang-migrate keeps no checksum, so editing applied migrations'
comments is safe.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/migrations/0067_user_normalization_prefs.up.sql

@@ -1,4 +1,4 @@
--- Each user's loudness-normalization preference (M464 #4998). It is kept on
+-- Each user's loudness-normalization preference. It is kept on
 -- the server, not per device, so the web player, the Android app and the
 -- leveled stream for UPnP/Sonos all apply the same one.
 --