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Change HAIRPIN_MODE to hairpin-veth as default

**What this PR does / why we need it**:
Change the default HAIRPIN_MODE back to "hairpin-veth".

It was previously "promiscuous-bridge" in order to workaround a kernel bug which deadlocked the machine when hairpin-veth was used. (#27498)

After some thorough manual testing on ubuntu clusters, we feel confident now that the kernel bug is fixed so we should switch back to using hairpin-veth. This will allow us to clean up some ebtables rules that were put in place to make "promiscuous-bridge" work properly.

Once this change goes in, we need to carefully monitor our e2e tests to make sure the bug has not resurfaced.

**Release note**:
```release-note
In a GCE cluster, the default HAIRPIN_MODE is now "hairpin-veth".
```

/cc @freehan @prameshj 
/assign @roberthbailey