[roll] Roll fuchsia [build][fidl] Target LEGACY in tree

This CL changes the fidl_library.gni template to target LEGACY instead
of the default version (HEAD). LEGACY is like HEAD, but re-adds APIs
marked as @available(removed=N, legacy=true). See:

* Iaa3fecafdf52b7c0b29535940af2af0181161cec for the design.
* Iaa541d3044fdd84259299cb05966d878f2251888 for the implementation.

As a test, I added @available(removed=HEAD, legacy=true) to Start() in
sdk/fidl/fuchsia.camera/camera.fidl. Before this CL, fx build failed:

    ../../src/camera/drivers/usb_video/usb_video_stream.h:166:16: error: only virtual member functions can be marked 'override'
      void Start() override { CloseStreamOnError(SetStreaming(true), "start"); }

But with this CL's change, it succeeded, indicating it was successfully
building against LEGACY bindings.

Original-Fixed: 106549
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/739983
Original-Revision: 93203d6053a392f1f68a6a73ee53fea2c6300704
GitOrigin-RevId: ac91ada2b4b27a9dfa66d9f11ac6a3ca26a7ece0
Change-Id: I442bae7a32f2934f5c8c30c943c3a46348ffc691