Document that VeraCrypt PIM will only be supported in Tails 4.0

Users complain that the PIM is not supported in 3.9. We know that the PIM is only supported if cryptsetup 2.0 is installed, which will be the case in Tails 4.0. I suggest we add a note about this to the known issues of 3.9.

Parent Task: #14468

Original created by @segfault on 15927 (Redmine)

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