GitReleaseManager create creates multiple draft releases on multiple runs

[AdmiringWorm]

Description

When running GitReleaseManager multiple times, then multiple draft releases will be created instead of updated.

Expected Behavior

If there is an existing draft release with the same name, I expect the same draft release to be updated (or an exception to be thrown).

Actual Behavior

The draft release is ignored, and a new draft release is instead created (without even discarding the old one).

Possible Fix

Find the opened draft release, if one exist with the same name/milestone then update the draft release instead of creating a new one.

Steps to Reproduce

1. Run GRM create
2. Run GRM create
3. Check GitHub releases page

Context

I quite often have GRM running automatically on the master branch (when building on the CI), which sometimes results in multiple draft releases being created because I maybe had to fix a small issue before doing the actual release.

Your Environment

• Version Used: Can't remember, the one Cake.Recipe uses.
• Edition Used (.NET Core, .NET Framework): .NET Framework
• Operating System and version (Windows 10, Ubuntu 18.04): Windows (Appveyor)