util/mak: deprecate NonNil, add type-safe NonNilSliceForJSON, NonNilMapForJSON

And put the rationale in the name too to save the callers the need for a comment.

Change-Id: I090f51b749a5a0641897ee89a8fb2e2080c8b782
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f