chore(changelog): clarify array attributes impact on `beforeSend*` callbacks (#21186)

The last release shipped array attribute support, which can subtly break
user hooks if users have special treatment for these. Update the
changelog to be a bit more explicit about this behavioral change.

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Lukas Stracke <lukas.stracke@sentry.io>