Ruby: String.try_convert isn't value-preserving

`String.try_convert` can convert arbitrary objects to strings, which
obviously isn't value-preserving.

ruby/ql/lib/codeql/ruby/frameworks/core/String.qll

@@ -47,7 +47,7 @@ module String {
     override predicate propagatesFlowExt(string input, string output, boolean preservesValue) {
       input = "Argument[0]" and
       output = "ReturnValue" and
-      preservesValue = true
+      preservesValue = false
     }
   }