core[major]: Drop python 3.8 support (#25996)

Drop python 3.8 support as EOL is 2024 October

.github/scripts/check_diff.py

@@ -111,7 +111,7 @@ def _get_configs_for_single_dir(job: str, dir_: str) -> List[Dict[str, str]]:
             {"working-directory": dir_, "python-version": f"3.{v}"}
             for v in range(8, 13)
         ]
-    min_python = "3.8"
+    min_python = "3.9"
     max_python = "3.12"
 
     # custom logic for specific directories

.github/workflows/scheduled_test.yml

@@ -17,7 +17,7 @@ jobs:
       fail-fast: false
       matrix:
         python-version:
-          - "3.8"
+          - "3.9"
           - "3.11"
         working-directory:
           - "libs/partners/openai"