IndexRange dependency vulnerability

[Microsoft.Bcl.Memory](https://www.nuget.org/packages/Microsoft.Bcl.Memory/) is a transitive dependency of [IndexRange ](https://github.com/bgrainger/IndexRange/blob/master/src/IndexRange/IndexRange.csproj#L58) and recently there's been a [vulnerability identified ](https://github.com/advisories/GHSA-73j8-2gch-69rq) in `Microsoft.Bcl.Memory`.

I was initially going to make a PR to IndexRange to bump the transitive dependency but the repo is listed as unsupported and the [author has noted they aren't going to work on it anymore](https://github.com/bgrainger/IndexRange/commit/2199323510c930403b5c43b547ea6c6c77614dba).

Given that this dependency won't be reasonably updated I was wondering what the path forward here should be:
1. Pull in the code with original attribution to this repo
2. Fork the repo and begin publishing the dep under the `Corvus` banner

Or something else completely.

Given that the repo is under the MIT license shouldn't be any issues here just copying the code over, etc if that's the easiest path.

EDIT: I've gone ahead and [made the PR in `IndexRange` ](https://github.com/bgrainger/IndexRange/pull/16)just in case the owner is open to publishing a new version for this however I think the above question still stands.