[CI] Upgrade to GCC 11 to fix stablehlo template instantiation error

[Yu-Zhewen]

The GCC CI workflow was failing to build stablehlo due to a C++ template instantiation issue with GCC 9.

StablehloAggressiveFolder.cpp uses modern C++ patterns combining if constexpr with SFINAE-based type traits:

if constexpr (fold_unary::DirectFolderExists<Impl, APInt>::value) {
    return FoldIfImplemented<APInt>(operand); 
}

GCC 9 incorrectly evaluates these type traits eagerly before the if constexpr condition is resolved, causing "incomplete type" errors:

/__w/iree/iree/third_party/stablehlo/stablehlo/transforms/optimization/StablehloAggressiveFolder.cpp:501:48: error: no matching function for call to 'FoldIfImplemented<llvm::APInt>(llvm::APInt&)'

/__w/iree/iree/third_party/stablehlo/stablehlo/transforms/optimization/StablehloAggressiveFolder.cpp:464:78: error: incomplete type 'fold_unary::DirectFolderExists<Impl, llvm::APInt>' used in nested name specifier

Upgrade to GCC 11 resolves this problem.

Before: https://github.com/iree-org/iree/actions/runs/20654878139/job/59305852621

After: https://github.com/iree-org/iree/actions/runs/20660113060/job/59320617905?pr=23008

[kuhar]

Version bump sgtm, gcc11 came out almost 5 years ago.

Is there any documentation we maintain about supported tool chains that also needs updating?

[Yu-Zhewen]

I found two places referencing GCC versions:

1. docs/website/docs/developers/debugging/releases.md mentions gcc 9.3.1 for manylinux, but I believe releases are actually using clang-17 now (see this recent run). This doc appears to be outdated.

2. cpubuilder_ubuntu_jammy.Dockerfile, the Docker image used by this GCC CI installs gcc-9. We could update it to gcc-11, or simply remove it since Ubuntu Jammy provides gcc-11 by default.

I can address these in separate PRs.

[kuhar]

I think all of these should be updated together to keep a consistent state on gcc

[Yu-Zhewen]

I've updated the documentation in this PR and created iree-org/base-docker-images#39 for the Docker image change (separate repo). Once that PR is merged, I'll update the container SHA here to reference the new image.