Update old version references, remove 3.1.0 release errata, and reset sha1sums to zeroes.
The actual version from https://github.com/wxWidgets/wxWidgets/releases/tag/v3.1.0 with the correct SHA1 sums, a few other minor edits and without the line breaks which don't seem to be handled correctly in the release announcements.
Update dates, changes descriptions and release preparation instructions.
