Update onnx package version minimum to 1.16.0. (iree-org#17504)

As discovered on
iree-org#17476 (comment), this
is needed to avoid an error running `iree-import-onnx` on certain test
files:

> onnx.onnx_cpp2py_export.checker.ValidationError: Your model ir_version
10 is higher than the checker's (9).

If we care about supporting older onnx package versions, we could add
some fallback code to `iree-import-onnx`.

We may also want to update the version used in torch-mlir?
https://github.com/llvm/torch-mlir/blob/e0a5adb1db38b0072c44b87570bc530eb3b324ad/setup.py#L264

.github/workflows/ci.yml

@@ -621,7 +621,7 @@ jobs:
           pip install --upgrade pip
           # Pre-fetch optional deps that iree-compiler needs (but we constrain that
           # to not consult a package index).
-          pip install onnx>=1.15.0
+          pip install onnx>=1.16.0
           pip install --no-index -f $PWD -v iree-compiler[onnx]
           echo "Testing default compiler:"
           python -m iree.compiler._package_test

compiler/setup.py

@@ -467,7 +467,7 @@ def find_git_submodule_revision(submodule_path):
     ],
     extras_require={
         "onnx": [
-            "onnx>=1.15.0",
+            "onnx>=1.16.0",
         ],
     },
 )