Mark reference to `build_requires` as deprecated

[fschoenm]

The documentation at https://docs.conan.io/2/reference/conanfile/methods/requirements.html mentions build_requires. If I remember correctly, build_requires was deprecated in Conan 2.x. Maybe it should be marked as deprecated there.

[memsharded]

Thanks for reporting

We are not yet in 2.X, but in 2.0. There is a current note explaining the status:

build_requires
--------------

`build_requires` are used in Conan 2.0 to provide compatibility with the Conan 1.X syntax,
but their use is discouraged in Conan 2 and will be deprecated in future 2.X releases.
Please use `tool_requires` instead of `build_requires` in your Conan 2 recipes.

The comment in https://docs.conan.io/2/reference/conanfile/methods/requirements.html is necessary to explain what is the behavior and traits assigned by that legacy method.

I have also pushed https://github.com/conan-io/docs/commit/1175d861d10a89cc8ad72a63e9742ca9e8ab18fe to remove the only legacy usage I could find in the docs.