Bump Standlone Python from 20240415 -> 20240726

- Introduces minimum platform version 0.3.20 since stripped versions of
  Standalone Python only exist for 20240726 and later.

{{ cookiecutter.format }}/briefcase.toml

@@ -1,14 +1,17 @@
 # Generated using Python {{ cookiecutter.python_version }}
+[briefcase]
+target_version = "0.3.20"
+
 [paths]
 app_path = "src/app"
 app_requirements_path = "requirements.txt"
 support_path = "support"
 {{ {
-    "3.8": 'support_revision = "3.8.19+20240415"',
-    "3.9": 'support_revision = "3.9.19+20240415"',
-    "3.10": 'support_revision = "3.10.14+20240415"',
-    "3.11": 'support_revision = "3.11.9+20240415"',
-    "3.12": 'support_revision = "3.12.3+20240415"',
+    "3.8": 'support_revision = "3.8.19+20240726"',
+    "3.9": 'support_revision = "3.9.19+20240726"',
+    "3.10": 'support_revision = "3.10.14+20240726"',
+    "3.11": 'support_revision = "3.11.9+20240726"',
+    "3.12": 'support_revision = "3.12.4+20240726"',
 }.get(cookiecutter.python_version|py_tag, "") }}