[Tracking] Clean up item related to Tokenizers

In the PR https://github.com/dotnet/machinelearning/pull/7264 we have added dependency on the `Microsoft.Bcl.Memory` which was not released as GA yet. This issue is tracking the tracking to clean up the following two items:

- `Microsoft.Bcl.Memory` will produce a warning in any project targeting net6. As we still have some projects targeting net6, we had to add the property `true` in the projects to avoid the warning. When we do the retargeting/upgrade such projects, we need to remove the added property.
- As `Microsoft.Bcl.Memory` not released and we are referencing the preview version of this package, we need to change that after the package became GA.