Update the TrueCrypt wrapper to make the migration path clearer

Once we have cryptsetup 1.6 and documentation about it, we shall update the TrueCrypt wrapper to point to that documentation, and to make it clear that TrueCrypt will be removed in Tails 1.3.

Feature Branch: feature/5373-replace-truecrypt

Parent Task: #5373 (closed)

Related issues

Blocked by #6052 (closed)
Blocked by #5932 (closed)
Blocks #7740 (closed)

Original created by @intrigeri on 7739 (Redmine)

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