Update robots.txt disallow to allow users/groups starting with "s"

## What does this MR do?

Update robots.txt "Disallow: /s" line to "Disallow: /s/"

## Why was this MR needed?

Currently any Project that is under a username or group name that starts with "s" cannot be indexed because of this disallow line.

For example on https://gitlab.com looking at the projects with the most stars there is this project: https://gitlab.com/somerobots/Trident

If you do a Google search for "site:gitlab.com Trident", the project comes up but with "A description for this result is not available because of this site's robots.txt"

Adding the trailing slash will still disallow all snippet short urls, but start allowing projects where the URI begins with "s"

See merge request !6481