log/slog: use encoding.TextAppender for text handlers

Fixes #79672

Change-Id: I0b77742104708d481ee52d16643d0aba6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/783340
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>