Multipath support not enabled in kernel

**Is your feature request related to a problem? Please describe.**
Please enable multipath support

```
[kevchaud@kevchaud-8QH6043 ~]$ sudo ip netns add INET
[kevchaud@kevchaud-8QH6043 ~]$ sudo ip netns exec INET ip link set dev lo up
[kevchaud@kevchaud-8QH6043 ~]$ sudo ip netns exec INET ip route add default nexthop via 41.10.0.1 nexthop via 41.11.0.1
Error: Multipath support not enabled in kernel.
[kevchaud@kevchaud-8QH6043 ~]$ uname -r
5.15.90.1-microsoft-standard-WSL2
```

**Describe the solution you'd like**
Update config: `CONFIG_IP_ROUTE_MULTIPATH=y`

**Describe alternatives you've considered**
I have built a custom kernel by modifying the instructions here https://perso.uclouvain.be/tom.barbette/mptcp-on-windows-with-wsl2/ but it would be nice to not have to do that.

**Additional context**
None

Start by locating the WSL kernel configuration and checking how CONFIG_IP_ROUTE_MULTIPATH is currently set. Enable CONFIG_IP_ROUTE_MULTIPATH=y, build or validate the kernel using the repository’s documented process, and confirm that the reported multipath route command no longer returns the kernel-support error.

Written by the indexing model from the issue text.