Update flatbuffers to v24.3.25 (#3966)

Summary:
See changes in https://github.com/google/flatbuffers/compare/v23.5.26...v24.3.25

We don't need anything specific from this newer version, but it's good to stay up-to-date to ensure we have the latest bug/security fixes.

This required updating the buck targets to match the latest source/header lists at https://github.com/google/flatbuffers/blob/v24.3.25/CMakeLists.txt#L121-L207

Pull Request resolved: https://github.com/pytorch/executorch/pull/3966

Test Plan:
Followed the instructions at https://pytorch.org/executorch/main/getting-started-setup.html

`buck2 build //examples/portable/executor_runner:executor_runner` succeeds. This builds and runs `flatc` from buck as a side-effect.

Reviewed By: lucylq

Differential Revision: D58493192

Pulled By: dbort

fbshipit-source-id: 8023c52a370a2849d0422375ab891ba8ec7cc7b9