Fix compatibility with ftxui-7.0.0 system packages. FTXUI's CMake package
uses SameMajorVersion compatibility, so requesting 6.0.2 rejected an
installed 7.0.0 and fell back to FetchContent, which fails on offline
builders like FreeBSD ports.

Fixes https://github.com/ArthurSonzogni/json-tui/issues/38