Use AsSpan/AsMemory (start, length) (#45310)

Use AsSpan/AsMemory (start, length)  (#45310)

* Use AsSpan(start, length)

Replace `AsSpan().Slice(start, length)` with `AsSpan(start, length)`

* Use AsMemory(start, length)

Replace `AsMemory().Slice(start, length)` with `AsMemory(start, length)`

* Apply suggestions from code review

Co-authored-by: Stephen Toub <stoub@microsoft.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>