Remove `--preview` from CLI commands

[willtsai]

NOTE: This should only be implemented after v0.61 is released

When compute extensibility is ready to launch, the --preview tags should be removed from the CLI commands, meaning that all the imperitive commands (e.g. rad environment create, etc.) should all operate on the new Radius.* resource types.

Support for the legacy Applications.* resource types should remain, but only via declarative operations (e.g. rad deploy app.bicep should still work if the app.bicep contains Application.* resources)

[nicolejms]

Current implementation and remaining work

Implemented:

• Radius v0.61.0 shipped in #13107, satisfying this issue's release prerequisite.
• Radius.Core imperative coverage continued through #12728 (rad resource list).

Remaining:

• The CLI still wires Radius.Core behavior behind --preview/RADIUS_PREVIEW in cmd/rad/cmd/root.go, with legacy behavior as the default. Remove that routing and make Radius.* the default for imperative commands while retaining declarative Applications.* deployments.

[radius-triage]

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