Prepare release notes and such for 3.0.0-rc2.

No real changes, just change the RC number.

Also update the release date.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75081 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Update checksums for the 3.0.0-rc2 release files.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75082 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775