Remove SliceHeader and replace.

[nicolasgarza]

Removes most uses of SliceHeader as referenced by #8422

SliceHeader is deprecated - so we prefer to use unsafe.Slice() and unsafe.Pointer().

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[ayushr2]

Also please squash all your commits:

gvisor/CONTRIBUTING.md

Lines 95 to 99 in 63343ff

	When iterating on a pull request, please amend your commit rather than creating
	new commits on the same branch. Our merge bot applies all commits from the
	branch to master and does not yet support squash merging. If you'd like to
	submit a pull request with multiple commits, please ensure they are independent,
	logical units of work that can be applied separately.

[ayushr2]

Thanks for your contribution!