add gocritic/ruleguard checks back again, cleanup code. (#13665)

- remove some duplicated code
- reported a bug, separately fixed in #13664
- using strings.ReplaceAll() when needed
- using filepath.ToSlash() use when needed
- remove all non-Go style comments from the codebase

Co-authored-by: Aditya Manthramurthy <donatello@users.noreply.github.com>

cmd/gateway/gcs/gateway-gcs.go

@@ -531,7 +531,7 @@ func toGCSPageToken(name string) string {
 		byte(length & 0xFF),
 	}
 
-	length = length >> 7
+	length >>= 7
 	if length > 0 {
 		b = append(b, byte(length&0xFF))
 	}