openconnect: Adjust version check in CMake and allow version 3.99 again

Commit 4cb6ba1b ("[openconnect] Simplify CMake code") mentioned removing support for openconnect < 3.99, which was also the case for the CMake code that got removed.

The commit, however, also excluded version 3.99 itself in the pkg-config version check, likely by accident. Restore the previous version check by allowing 3.99 again.