[Core] Don't force uppercase for VLLM_LOGGING_LEVEL (#15306)

Signed-off-by: Russell Bryant <rbryant@redhat.com>

vllm/envs.py

@@ -294,7 +294,7 @@ environment_variables: dict[str, Callable[[], Any]] = {
 
     # this is used for configuring the default logging level
     "VLLM_LOGGING_LEVEL":
-    lambda: os.getenv("VLLM_LOGGING_LEVEL", "INFO"),
+    lambda: os.getenv("VLLM_LOGGING_LEVEL", "INFO").upper(),
 
     # if set, VLLM_LOGGING_PREFIX will be prepended to all log messages
     "VLLM_LOGGING_PREFIX":