Declare that Windows XP and earlier are not officially supported.

The actual cleanup parts of this are at legacy/trac#11444 (moved) and legacy/trac#11445 (moved).

To be specific, Windows XP and below are officially EOL and support for those targets may break in the future. This fact should be documented formally somewhere.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information