fix: `after` should show the same ttl as the rate limit headers

[gurgunday]

Simplifies the rate limiter by only looking at the ttl to construct the after message — this aligns the message with the Retry-After header; before, it would just send the timeWindow, which is inaccurate

So even if a user had 5 seconds left, if the timeWindow was 50 seconds, the error would say, "Retry in 50 seconds" and not "Retry in 5 seconds"

[gurgunday]

I will rebase after https://github.com/fastify/fastify-rate-limit/pull/393 merges