Updates to support 4.0.0.0-preview release.

[boblodgett]

Description

These changes update the generator and extensions to support 4.0.0.0-preview versions and the initial preview release.

• DevConfig file added to override the version to 4.0.0.0.
• Preview flag is set in the _sdk-versions.json file.
• Added license/notice.txt files to net462 and net8.0 targets

Motivation and Context

Testing

• Development account E2E tests succeeded.
• Release artifacts tested.
• Pipeline inspected to ensure proper function.

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license