What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Changes kind-up.sh to warn about potential MTU/PMTU issues with Docker networking on Linux instead of attempting to run the iptables workaround command directly.

Previously, the script attempted to run:

iptables -t mangle -A POSTROUTING -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

This approach had several problems:

1. Requires sudo, which causes file ownership issues (e.g., files created during make kind-up end up owned by root)
2. Not everyone will encounter MTU/PMTU issues - it depends on their network configuration
3. The iptables rule only needs to be applied once per reboot, not every time the script runs

Now the script simply prints a warning message pointing users to the relevant GitHub issue if they experience network timeouts.

Which issue(s) this PR fixes:

NONE

Special notes for your reviewer:

The original issue (kubernetes/test-infra#23741) was a GKE-specific networking bug from 2021 that has since been resolved. The workaround may still be useful for some nested Docker networking scenarios, but it's not universally needed.

Does this PR introduce a API change?

NONE

Additional documentation e.g., enhancement proposals, usage docs, etc.:

NONE