datapath: Use SHA256 instead of SHA1 for datapath hash

[twpayne]

SHA1 is probably fine for hashing the datapath contents, but it will likely raise a flag in any security reviews. In anticipation of this happening in the future, switch to SHA256 instead.

This PR removes the last direct use of SHA1 by Cilium so we can remove the resumable SHA1 package too.

cc @sharlns as raised by @rolinh in the SIG Security meeting today.

[twpayne]

test-me-please

[joestringer]

New implementation looks functionally identical, we lost a couple of code comments and some unit testing but I don't think it's a loss. Somehow the old unit test code didn't even seem to test the implementation in the package anyway.

[rolinh]

Thanks! I was a little worried about possible side-effects of changing the hash type (e.g. in the context of an upgrade) but apparently all objects are reloaded anyway in such a scenario.