update powershell-preview to 7.0.0-preview.1 (#7436)
update powershell-preview to 7.0.0-preview.1 (#7436)

* update powershell-preview to 7.0.0-preview.1
* used lowercase sha256 alpha chars in checksum value
404NetworkError/homebrew-versions,mahori/homebrew-cask-versions,Ngrd/homebrew-versions,caskroom/homebrew-versions,caskroom/homebrew-versions,lantrix/homebrew-versions,rogeriopradoj/homebrew-versions,pkq/homebrew-versions,victorpopkov/homebrew-versions,lantrix/homebrew-versions,404NetworkError/homebrew-versions,rogeriopradoj/homebrew-versions,pkq/homebrew-versions,victorpopkov/homebrew-versions,Ngrd/homebrew-versions