Announce in advance that 3.0 will be 64-bit only

We should communicate this early enough to our users, so that they can plan any needed hardware upgrade accordingly. I’d like to do during the 2017-01 sprint. I’ll draft something; it would be awesome if sajolida could review and improve it, but not a strict requirement.

Subtasks

#12192 (closed)
#12193 (closed)

Related issues

Related to #8183 (closed)

Original created by @intrigeri on 12163 (Redmine)

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