log/slog: use Infinity instead of Inf

log/slog: use Infinity instead of Inf

JSON is derived from Javascript, so we should use Javascript-inspired
literals instead of ones more common to Go.

In Javascript, infinity is declared as Infinity rather than Inf.

Change-Id: I6c81353d0c677640f3f11961a37d792408ac03fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/478758
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>