Inaccurate description and issue/PR link in Preview 7: Object Stack Allocation for Boxes section

### URL(s)

https://github.com/dotnet/core/blob/main/release-notes/9.0/preview/preview7/runtime.md#object-stack-allocation-for-boxes

### Description

The description appears to link to an issue which outlines *upcoming* object stack allocation / escape analysis work in .NET 10.

Expected link: https://github.com/dotnet/runtime/pull/103361

Actual link: https://github.com/dotnet/runtime/issues/104936

In addition, the description misses important detail - .NET was previously capable of limited object stack allocation but due to unprofitability of the feature, as implemented in scope of https://github.com/dotnet/runtime/issues/11192, it was left disabled.

The work conducted in scope of https://github.com/dotnet/runtime/pull/103361 tuned compiler heuristics to improve that, and changed object stack allocation to be enabled by default: https://github.com/dotnet/runtime/pull/103361/files?diff=unified&w=0#diff-9e3a8cb6a407916a040efe2d7e973e6e86e4d3d44acd4c4f54ce470d3540dc80L657-R658

Recently, I wrote a post that discusses the above in more detail which you can find here: https://news.ycombinator.com/item?id=41211172

Thanks!