Update LICENSE.txt so that it's recognized as MIT

[xtqqczze]

• Update LICENSE.txt so that Github recognizes it as MIT.
• Remove the term All Rights Reserved as Microsoft no longer recommend it.

See also: https://github.com/PowerShell/PowerShell/pull/12729

Pull Request Creation Checklist

• [x] Useful title (i.e. Fix #XXX: Description / Feature #XXX: Description / RFC #XXX: Description)
• [x] Description of the changes you are making
• [x] Added a link to the related Github issue in the description (i.e. type Fixes #XXX in the desciption of the PR)
• [x] If you are still working on the code and would like some early feedback via the Pull Request process just add [WIP] to the beginning of your Pull Request title
• [x] Checked the Polaris GitHub workflow guidance to make sure my code has the latest changes made to the master branch
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