Change Regex::MatchData#to_s to return matched substring

[Vendicated]

Related issue: #13118

Currently, MatchData#to_s is the same as MatchData#inspect, which means that it returns an object representation of the MatchData: Regex::MatchData("ys" 1:"y" 2:"s")

This is not only inconsistent with Ruby, which returns the matched substring, and most other languages, like rust's regexp crate, but also not really useful for anything besides debugging (which is exactly where you should use inspect over to_s).

Much more useful would be for it to return the matched substring. Currently, the only way to do so is via match[0], but that's unintuitive and confusing

Thus, this PR changes #to_s to do just that. This is a breaking change, but I doubt many codebases actually use or rely on the old to_s. #inspect still has the old behaviour.

Resolves part of #13118

[Vendicated]

CI fail seems to be unrelated

     1) UDPSocket using IPv4 joins and transmits to multicast groups
  
         Could not bind to '0.0.0.0:37747': Address already in use (Socket::BindError)