Issue cleanup automation should close issues "As not planned" instead of "As completed"

[julealgon]

The issue cleanup bot, running under dotnet-policy-service, currently closes issues with the standard "Completed" (purple) reason.

Issues marked as "Completed" are meant to be used when something was done to fix/implement them, which is not the case when the closure reason was staleness.

Here is an example:

• MEF on .NET Core 2.1 fails to load plugins from folder , but works well with .NET Framework 4.6.1 #30978

I propose that the automation is changed to use the "Close as Not Planned" reason (gray) instead, to make it clearer that the issue was not implemented.

Some similar discussion here for reference:

• Support the "not planned" state reason (when available) actions/stale#744 (comment)

There, it is even mentioned that "Close as Not Planned" hints as "staleness" being one of the possible underlying use cases:

[dotnet-policy-service]

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