.NET Core May 2021 Update - .NET 5.0.300 SDK

Today we are releasing .NET 5.0.300 SDK as a supported SDK with the release of [Visual Studio 16.10](https://docs.microsoft.com/visualstudio/releases/2019/release-notes#16.10.0)

Release Notes
[5.0.300 SDK](https://github.com/dotnet/core/blob/master/release-notes/5.0/5.0.6/5.0.300-sdk.md)

Please report any issues you find with .NET 5.0.300 SDK, by responding to this issue.

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Status of availability of SDK on Microsoft's Linux feeds:

| Distro | 5.0.300 |
|--|--|
| Ubuntu 16.04 | ✅ |
| Ubuntu 18.04 | ✅ |
| Ubuntu 20.04 | ✅ |
| Ubuntu 20.10 | ✅ |
| Centos 7 |✅ |
| Debian 9 | ✅ |
| Debian 10 | ✅ |
| Fedora 32 | ✅ |
| Fedora 33 | ✅ |
| OpenSUSE 15 | ✅ |
| SLES 12 | ✅ |
| SLES15 | ✅ |

Note: This list refers to the Microsoft-provisioned feeds (packages.microsoft.com) and does not in any way represent direct availability in distros (eg RHEL, Fedora).

### Known Issues
If there are any issues with this release we will track them here and check issues off as they're resolved. See the linked issues for details on progress and resolution details.