[Sftp] Remove 'preview' tag to fix version

[VeryEarly]

Description

Mandatory Checklist

• Please choose the target release of Azure PowerShell. (⚠️Target release is a different concept from API readiness. Please click below links for details.)

  • General release
  • Public preview
  • Private preview
  • Engineering build
  • No need for a release

• Check this box to confirm: I have read the Submitting Changes section of CONTRIBUTING.md and reviewed the following information:

• SHOULD update ChangeLog.md file(s) appropriately
  • Update src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md.
    • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header in the past tense.
  • Should not change ChangeLog.md if no new release is required, such as fixing test case only.
• SHOULD regenerate markdown help files if there is cmdlet API change. Instruction
• SHOULD have proper test coverage for changes in pull request.
• SHOULD NOT adjust version of module manually in pull request

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[Copilot]

Pull Request Overview

This PR removes the 'preview' tag from the Az.Sftp module to transition it from preview status to general availability. The changes enable proper module versioning by commenting out the prerelease designation and adding the module to the CreateMappings configuration.

• Removes the 'Prerelease = preview' setting from the module manifest
• Adds Sftp module mapping to the CreateMappings_rules.json configuration

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Sftp/Sftp/Az.Sftp.psd1	Comments out the Prerelease property to remove preview status
tools/CreateMappings_rules.json	Adds Sftp module alias mapping configuration