GitVersion: Add ShortSha property

[nils-a]

This came up in https://github.com/orgs/cake-build/discussions/3915.

GitVersion has a ShortSha property since version 5.0.0-alpha1 (i.e. late 2018).

[cake-build-bot]

🎉 This issue has been resolved in version v2.3.0 🎉

The release is available on:

• GitHub Release
• .NET Tool

Your GitReleaseManager bot 📦🚀