Remove net6.0;net7.0 from System.Numerics.Tensors

[ericstj]

In #92269 we re-enabled net6.0;net7.0 to make it easier to backport changes from main to 8.0. We should remove those before we ship 9.0 since these frameworks will be out of support by the time it ships. cc @ViktorHofer

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Issue Details

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In #92269 we re-enabled net6.0;net7.0 to make it easier to backport changes from main to 8.0. We should remove those before we ship 9.0 since these frameworks will be out of support by the time it ships. cc @ViktorHofer

Author:	ericstj
Assignees:	-
Labels:	area-System.Numerics.Tensors, untriaged, needs-area-label
Milestone:	-