Remove Unstable Trait from Cloudformation traits

[prestomation]

aws.cloudformation#cfnExcludeProperty, aws.cloudformation#cfnResource, and aws.cloudformation#cfnMutability are marked as unstable, but I suspect these are used broadly. Are they really unstable? I can suppress these, but there are lots of warnings in my model that can make it difficult to notice new, unrelated-to-cloudformation warnings.

[kstich]

We'll need to investigate if/how other alterations to CloudFormation Resource Schema generation are being made to validate these are the stable shapes for these traits. Should include resolution on #923 to do this work.