Upgrade flatbuffers version to 23.5.26 (#465)

Summary:
`23.5.26` from conda-forge https://anaconda.org/conda-forge/flatbuffers matches the third-party commit.  This is to avoid compilation issue when the version of `flatbuffers` from conda (used by CI) is different than the third-party commit (used when compiling).

Pull Request resolved: https://github.com/pytorch/executorch/pull/465

Reviewed By: cccclai

Differential Revision: D49567226

Pulled By: huydhn

fbshipit-source-id: 8926631eebf5faad3d0aaaa288fd369c18b472f7