chore: clarify .Net core versions in contributing guide (#20612)

I spent some time installing .Net Core 3.1.x on my Mac and struggling with
unsigned binaries before I discovered that the latest version works.


----

### All Submissions:

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

CONTRIBUTING.md

@@ -57,7 +57,7 @@ The following tools need to be installed on your system prior to installing the
 - [Node.js >= 14.15.0](https://nodejs.org/download/release/latest-v14.x/)
   - We recommend using a version in [Active LTS](https://nodejs.org/en/about/releases/)
 - [Yarn >= 1.19.1, < 2](https://yarnpkg.com/lang/en/docs/install)
-- [.NET Core SDK 3.1.x](https://www.microsoft.com/net/download)
+- [.NET Core SDK >= 3.1.x](https://www.microsoft.com/net/download)
 - [Python >= 3.6.5, < 4.0](https://www.python.org/downloads/release/python-365/)
 - [Docker >= 19.03](https://docs.docker.com/get-docker/)
   - the Docker daemon must also be running