Update vCPE example for JSON Stringify

Update vCPE 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 vCPE example in apex to use that new method.

Issue-ID: POLICY-2463
Change-Id: I85cc33dc4633b1b6f4c4db3d5ad8579c66d7c18c
Signed-off-by: liamfallon <liam.fallon@est.tech>
(cherry picked from commit da12c1c622ebc2304965ff1f646dc9783c5363c6)