COMP: Add Dependabot for actions and raise Superbuild Python floor

Add a Dependabot configuration that proposes grouped weekly updates for
the GitHub Actions used in CI, so action versions stay current without
manual tracking (several were emitting Node-20 deprecation warnings).

Raise the Superbuild minimum Python from the end-of-life 3.7 to 3.11 to
match the version used throughout the CI matrix and supported by current
ITK.

Author: hjmjohnson

.github/dependabot.yml

@@ -0,0 +1,10 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    groups:
+      actions:
+        patterns:
+          - "*"

Superbuild/External-Python.cmake

@@ -1,4 +1,4 @@
-set(MIN_PYTHON_VERSION 3.7)
+set(MIN_PYTHON_VERSION 3.11)
 find_package(Python3 ${MIN_PYTHON_VERSION} COMPONENTS Interpreter REQUIRED)
 
 set(_itk_venv "${CMAKE_CURRENT_BINARY_DIR}/itkpython")