Runtime libraries - version compatibility

**Help us make content visible**

Take this article, for instance: https://docs.microsoft.com/es-es/dotnet/standard/runtime-libraries-overview

**Describe the new article**

It's not clear whether `Microsoft.Extensions.*` package versions have to be "aligned" (less or equal) to the runtime version.

E.g., is it ok to add a reference to `Microsoft.Extensions.* Version="6.0.0"` (targeting `netstandard2.0`)
from my `net48` or `netcoreapp3.1` projects? Are these newer packages backwards-compatible with older runtimes?

---
#### Document Details

⚠ *Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.*

* ID: 3cbdde85-09dd-c0c5-724c-50cee3c4da18
* Version Independent ID: 9a3e1a10-1b76-69ae-e3f5-2e030aceb351
* Content: [Runtime libraries overview](https://docs.microsoft.com/en-us/dotnet/standard/runtime-libraries-overview)
* Content Source: [docs/standard/runtime-libraries-overview.md](https://github.com/dotnet/docs/blob/main/docs/standard/runtime-libraries-overview.md)
* Product: **dotnet-fundamentals**
* GitHub Login: @tdykstra
* Microsoft Alias: **gewarren**