docs: .NET Core 3.1 is required with JSII v1.1 (#6951)

BREAKING CHANGE: .NET Core v3.1 is required with JSII v1.1

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

README.md

@@ -20,7 +20,7 @@ The CDK is available in the following languages:
 * JavaScript, TypeScript ([Node.js ≥ 10.3.0](https://nodejs.org/download/release/latest-v10.x/))
 * Python ([Python ≥ 3.6](https://www.python.org/downloads/))
 * Java ([Java ≥ 8](https://www.oracle.com/technetwork/java/javase/downloads/index.html) and [Maven ≥ 3.5.4](https://maven.apache.org/download.cgi))
-* .NET ([.NET Core ≥ 3.0](https://dotnet.microsoft.com/download))
+* .NET ([.NET Core ≥ 3.1](https://dotnet.microsoft.com/download))
 
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