chore: remove note of go being in dev preview

[MrArnoldPalmer]

Removes note in go app init template readmes saying go is in dev preview
since it is not generally available. Also adds note about cdk.json which may
be useful for new users and is present in other language templates.

references: awsdocs/aws-cdk-guide#416

---

All Submissions:

• Have you followed the guidelines in our Contributing guide?

Adding new Unconventional Dependencies:

• This PR adds new unconventional dependencies following the process described here

New Features

• Have you added the new feature to an integration test?
  • 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

[gitpod-io]

[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: eb295f0
• Result: SUCCEEDED
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[mergify]

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