Update gRPC example for JSON Stringify

Update gRPC example for JSON Stringify

The JSON.stringify() call in the Rhino Javascript interpreter is not
working correctly. A previous patch introduced a method into the Apex
execution context to provide an alternative method for stringifying
JSON.

This patch updates the gRPC example in apex to use that new method.

Issue-ID: POLICY-2463
Change-Id: I7f789c000d53dec60020fda2a6fccd71ef18edc3
Signed-off-by: liamfallon <liam.fallon@est.tech>
(cherry picked from commit fd5ee076b2ecf9fc5151090cf8a9b49fac6cf055)