chromadb requested in AIPCC-10391 requires Rust 1.92.

Rust 1.92 requires LLVM 21 (specifically libLLVM.so.21.1(LLVM_21.1)(64bit)), we only have LLVM 20. To provide Rust 1.92, we need to:

• request llvm21 dist-git with branch rhelai-3.4-rhel-9 from SPRHEL,
• build LLVM 21 compat build in Brew,
• tag Rust 1.92 from RHEL 9.8 into rhelai-3.4-rhel-9-candidate,
• bump RUST_MIN_VERSION in builder.