Update CLI11 to v1.9.1

This CLI11 update from v1.9.0 to v1.9.1 provides the following improvements: 
- Support relative inclusion [#475](CLIUtils/CLI11#475)
- Fix cases where spaces in paths could break CMake support [#471](CLIUtils/CLI11#471)
- Fix an issue with string conversion [#421](CLIUtils/CLI11#421)
- Cross-compiling improvement for Conan.io [#430](CLIUtils/CLI11#430)
- Fix option group default propagation [#450](CLIUtils/CLI11#450)
- Fix for C++20 [#459](CLIUtils/CLI11#459)
- Support compiling with RTTI off [#461](CLIUtils/CLI11#461)