Fix robots.txt to disallow indexing of all paths (#10150)

Following suit of recent change logged in Consul hashicorp/consul#8958

The path pattern specified in allow / deny lines should start with a forward slash to designate the root.

[1] https://tools.ietf.org/html/draft-koster-rep-00#section-2.2
[2] https://developers.google.com/search/reference/robots_txt

ui/public/robots.txt

@@ -1,3 +1,3 @@
 # http://www.robotstxt.org
 User-agent: *
-Disallow: *
+Disallow: /