[Fix] Use C++20 standard consistently to resolve ODR violations

[Fix] Use C++20 standard consistently to resolve ODR violations

This commit resolves an ODR violations when compiling with -flto and
-Werror=odr [1]. The main project used a newer C++20 standard, while the
backward-cpp and simdutf libraries used an older C++11 standard. This
difference caused the linker to fail.

Setting C++20 standard in both libraries resolves the ODR issue.

Link: https://bugs.gentoo.org/962041