For release.

## Change
Use an `IReference<Boolean>` rather than a custom enum to represent an
optional bool value. The projection is much cleaner and scales better to
other value types.

###### Microsoft Reviewers: [Open in
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- [ ] I have signed the [Contributor License
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- [ ] I have updated the [Release Notes](../doc/ReleaseNotes.md).
- [ ] This pull request is related to an issue.

-----

Branch https://github.com/microsoft/winget-cli/tree/release-v1.28
created from the current HEAD.
###### Microsoft Reviewers: [Open in
CodeFlow](https://microsoft.github.io/open-pr/?codeflow=https://github.com/microsoft/winget-cli/pull/6019)

###### Microsoft Reviewers: [Open in
CodeFlow](https://microsoft.github.io/open-pr/?codeflow=https://github.com/microsoft/winget-cli/pull/6026)

WinGet is not in preview anymore, so tiny update in the docs.