It happens to be the major version reported by `wsl --version` on my system.

    NRR-WT1; wsl --version
    WSL version: 3.0.1.0
    Kernel version: 6.18.40.1-1
    WSLg version: 1.0.79
    MSRDC version: 1.2.7214
    Direct3D version: 1.611.1-81528511
    DXCore version: 10.0.26100.1-240331-1435.ge-release
    Windows version: 10.0.26300.9457
    NRR-WT1;