Deprecated Lambda runtime

Hey!

I did an automated check for deprecated Lambda runtimes and it seems this project uses the `dotnetcore2.0` runtime, which is [deprecated since May 2019](https://docs.aws.amazon.com/lambda/latest/dg/runtime-support-policy.html). If this is using a deprecated runtime, this means that users won't be able to deploy that specific section.

Using `grep -nr dotnetcore2.0 *`:

```
Serverless/Serverless Web App with React, DynamoDb, and X-Ray/serverless.template:18: "Runtime": "dotnetcore2.0",
Serverless/Serverless Web App with React, DynamoDb, and X-Ray/serverless.template:45: "Runtime": "dotnetcore2.0",
```

Inspect Serverless/Serverless Web App with React, DynamoDb, and X-Ray/serverless.template at the two Runtime entries identified by the grep output. Check the AWS Lambda runtime support policy before making the configuration update, then verify that neither deprecated reference remains and the template is still deployable.

Written by the indexing model from the issue text.