Add 6.8 kernel to CI build. Update patch versions of existing kernels

[ddelnano]

Summary: Add 6.8 kernel to CI build. Update patch versions of existing kernels

In addition to updating the kernels, I've removed the use of the gs://pixie-dev-public/kernel-build bucket. Please see the changes commit by commit to see them independently.

Relevant Issues: N/A

Type of change: /kind cleanup

Test Plan: bpf-build-all-kernels build should succeed