[fcuny]

Moves the guest kernel from 7.3 to 7.3.1, with kernel/update.sh.

Before merging, test the kernel with kernel/test/pr.py and this PR's number.

No config changes.

[fcuny]

Tested 7.3.1 from the kernel workflow's run for 2de6972, with kernel/test/pr.py and c2vm at v0.1.0-52-g5febeb8: everything passed.

Check
boots, and runs the checks within 300s	✅
uname -r is 7.3.1-c2vm	✅
/proc/sys is there	✅
sees 4 CPUs	✅
gets an address over DHCP	✅
fetches over HTTPS	✅
the root is the overlay	✅
the random device is virtio	✅
reads a -v mount	✅
no errors in dmesg	✅
make e2e: 13 passed	✅