Rh-che: Update .NET 2.0 stack image to version 2.1

[kevchu3]

Issue problem:
The referenced quay.io/openshiftio/che-centos_dotnet20 stack in assembly/fabric8-stacks/src/main/resources/stacks.json is out of date as .NET 2.0 was deprecated in October 2018 [1]. I'm opening this issue to have both the quay.io image and this stack's reference to the image updated to .NET 2.1 (or 2.2). I'd expect to keep the naming convention, so perhaps quay.io/openshiftio/che-centos_dotnet21 (or dotnet22) would suffice.

Red Hat Che version:

che.openshift.io version: 7.0.0-RC-1.1
quay.io/openshiftio/che-centos_dotnet20 image version: 2.0.3

• [X] I can reproduce it on latest official image

Reproduction Steps:

Running dotnet --version from within the stack's terminal shows it's running version 2.0.3.

Runtime:

runtime used:

• [X] OpenShift.io

[amisevsk]

The stacks.json file will be deprecated completely once https://github.com/redhat-developer/rh-che/pull/1481 is merged; currently it is not used and instead the devfile is used instead (which does reference .NET 2.2)

[ibuziuk]

We are going to close this issue once che.openshift.io is updated to Che 7 GA