ext: Update pybind11 to version 2.6.2.

This should help reduce warning spew when building with newer compilers.
The pybind11::module type has been renamed pybind11::module_ to avoid
conflicts with c++20 modules, according to the pybind11 changelog, so
this CL also updates gem5 source to use the new type. There is
supposedly an alias pybind11::module which is for compatibility, but we
still get linker errors without changing to pybind11::module_.

Change-Id: I0acb36215b33e3a713866baec43f5af630c356ee
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40255
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

ext/pybind11/tests/test_docstring_options.cpp

@@ -45,6 +45,14 @@ TEST_SUBMODULE(docstring_options, m) {
 
     m.def("test_function7", [](int, int) {}, py::arg("a"), py::arg("b"), "A custom docstring");
 
+    {
+        py::options options;
+        options.disable_user_defined_docstrings();
+        options.disable_function_signatures();
+
+        m.def("test_function8", []() {});
+    }
+
     {
         py::options options;
         options.disable_user_defined_docstrings();