chore: start cleaning up 3.9 usage from examples (#3419)

Summary:
- **example: stop using 3.9 in multi-python-versions**
- **chore: cleanup unused code**

There is more to go, but this is a good start remaining of #2704

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

examples/multi_python_versions/WORKSPACE

diff --git a/examples/multi_python_versions/WORKSPACE b/examples/multi_python_versions/WORKSPACE
index 6b69e0a..0b6b8a0 100644
--- a/examples/multi_python_versions/WORKSPACE
+++ b/examples/multi_python_versions/WORKSPACE
@@ -9,15 +9,17 @@
 
 py_repositories()
 
-default_python_version = "3.9"
+default_python_version = "3.10"
 
 python_register_multi_toolchains(
     name = "python",
     default_version = default_python_version,
     python_versions = [
-        "3.9",
         "3.10",
         "3.11",
+        "3.12",
+        "3.13",
+        "3.14",
     ],
     register_coverage_tool = True,
 )
@@ -30,12 +32,16 @@
     python_interpreter_target = {
         "3.10": "@python_3_10_host//:python",
         "3.11": "@python_3_11_host//:python",
-        "3.9": "@python_3_9_host//:python",
+        "3.12": "@python_3_12_host//:python",
+        "3.13": "@python_3_13_host//:python",
+        "3.14": "@python_3_14_host//:python",
     },
     requirements_lock = {
         "3.10": "//requirements:requirements_lock_3_10.txt",
         "3.11": "//requirements:requirements_lock_3_11.txt",
-        "3.9": "//requirements:requirements_lock_3_9.txt",
+        "3.12": "//requirements:requirements_lock_3_12.txt",
+        "3.13": "//requirements:requirements_lock_3_13.txt",
+        "3.14": "//requirements:requirements_lock_3_14.txt",
     },
 )