This PR bumps the FlatBuffers version used by the backend to `24.3.25` (the latest version as of now).
Since the newer FlatBuffers releases come with prebuilt binaries for all platforms we target, we can simplify the build process by simply downloading the required `flatc` binary from the official FlatBuffers GitHub release page. This allows us to remove the dependency on `conda`, which was the only reliable way to get the outdated `flatc`.
The `conda` setup has been removed from the CI steps and the relevant code has been removed from the build script.
The FlatBuffers version is no longer hard-coded in the Rust build script, it is inferred from the `build.sbt` definition (similar to GraalVM).
# Important Notes
This does not affect the GUI binary protocol implementation.
While I initially wanted to update it, it turned out farly non-trivial.
As there are multiple issues with the generated TS code, it was significantly refactored by hand and it is impossible to automatically update it. Work to address this problem is left as [a future task](https://github.com/enso-org/enso/issues/9658).
As the Flatbuffers binary protocol is guaranteed to be compatible between versions (unlike the generated sources), there should be no adverse effects from bumping `flatc` only on the backend side.
