Core Bonsai packages should use strict version relationships

[PathogenDavid]

Right now the Bonsai bootstrapper package has >= version requirements against its core dependencies.

(For example, Bonsai 2.8.4 relies on Bonsai.Core >= 2.8.4)

These core packages were never really meant to be mixed-and-matched, and as of #1873 they're always versioned and released together.

As such they should use strict version checks, IE: Bonsai 2.8.4 should rely on Bonsai.Core =2.8.4.

Blocked by #1042

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As an aside, requiring exact versions for ProjectReferences is currently a little clunky, here's an example that could be adapted to only apply to core project references from core projects: NuGet/Home#5556 (comment)