Add rule to suggest `net5.0` target for v1.0 release

[augustoproiete]

With Cake v1.0 release, I believe we'll be recommending addins to multi-target net5.0, netstandard2.0, and net461 for broader reach.

It seems it would be an update to CCG0007.

[AdmiringWorm]

Just a note, should maybe take the cake version into account as well.

If the project references a version of cake below 1.0.0, only suggest netstandard and net461, but otherwise suggest all three (not sure if it is possible though).

[nils-a]

Yes, I planned on that.

[nils-a]

probably related to #91

[cake-contrib-bot]

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