.NET Core `latest` Docker tags updated to 3.1

# .NET Core `latest` Docker tags updated to 3.1

The `latest` tags for the [dotnet/core](https://hub.docker.com/_/microsoft-dotnet-core) repos have been updated to reference [.NET Core 3.1](https://devblogs.microsoft.com/dotnet/announcing-net-core-3-1/).

## Discussion

You can share your thoughts on this change or see what others are saying at:

* [dotnet/dotnet-docker#1337](https://github.com/dotnet/dotnet-docker/issues/1511)

## Details

* [dotnet-docker#1509](https://github.com/dotnet/dotnet-docker/pull/1509)

The `latest` tags for the [dotnet/core](https://hub.docker.com/_/microsoft-dotnet-core) repos are always intended to reference the latest supported version of .NET Core. At this time, the latest supported version is .NET Core 3.1. This version of .NET Core is also a [long-term supported (LTS) release](https://dotnet.microsoft.com/platform/support/policy/dotnet-core).

The following Docker repos have been updated:

* [dotnet/core/sdk](https://hub.docker.com/_/microsoft-dotnet-core-sdk)
* [dotnet/core/aspnet](https://hub.docker.com/_/microsoft-dotnet-core-aspnet)
* [dotnet/core/runtime](https://hub.docker.com/_/microsoft-dotnet-core-runtime)
* [dotnet/core/runtime-deps](https://hub.docker.com/_/microsoft-dotnet-core-runtime-deps)

It is recommended to use a more-specific tag than `latest` for anything other than casual development. Once you have an application that you believe will eventually go to production, you are recommended to use a tag like `3.1`.

Start with the linked dotnet-docker pull request #1509 and the four Docker repositories listed in the announcement. Verify that their latest tags reference .NET Core 3.1; the announcement already indicates that this update is complete.

Written by the indexing model from the issue text.