targets: match Pico2 stack size to Pico

Took me a while to debug weird crashes after switching from Pico to
Pico2.

Author: eliasnaur

targets/pico2.json

@@ -3,5 +3,6 @@
         "rp2350"
     ],
     "build-tags": ["pico2"],
-    "serial-port": ["2e8a:000A"]
+    "serial-port": ["2e8a:000A"],
+    "default-stack-size": 8192
 }