Avoid substring allocations in WebUtility.HtmlDecode (#29402)

* Avoid substring allocations in WebUtility.HtmlDecode

* Update changes to HtmlDecode based on feedback

- Use regular Dictionary instead of LowLevelDictionary
- Use AsSpan overload instead of AsSpan() and Slice
- Avoid shift by variable amount
- Use helper method to generate keys to make the code easier to maintain

* Assert that entity length is <= 8 in ToUInt64Key

- Add assert to ToUInt64Key
- Replace default with 0