Default PYTHONSTARTUP to opt out/off for Stable Release (#24140)

From discussion in the original issue:
#23930 (comment)
Making default to be false for September stable, perhaps we could turn
it on to true for insiders AFTER once we ship out stable.
/cc @Tyriar

Author: anthonykim1

package.json

@@ -658,7 +658,7 @@
                     "type": "array"
                 },
                 "python.REPL.enableShellIntegration": {
-                    "default": true,
+                    "default": false,
                     "description": "%python.REPL.enableShellIntegration.description%",
                     "scope": "resource",
                     "type": "boolean"