Replace remaining references to 2.9.0 with 2.9.1.

Replace remaining references to 2.9.0 with 2.9.1.

Also update the tech note listing the places that need to be changed when a
new version is released and the script which updates them automatically.

Finally update a couple of copyright years.

Closes #12185.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64913 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775