Additions:

• Add tentative C++23 detection macro gsl_CPP23_OR_GREATER, CONSTEXPR_23 language feature, and gsl_constexpr23 keyword macro (#329)
• Add gsl_CONFIG_VALIDATES_UNENFORCED_CONTRACT_EXPRESSIONS config switch for suppressing the compile-time syntax validation of contract check expressions (#330)

Changes:

• Deprecate string_span and zstring_span and prepare for removal (#335)
• Continuous Integration now also tests GCC 12, Clang 14 thru 16, and NVCC 11.8 and 12.1
• Remove Continuous Integration for GCC 6 and earlier, Clang 5 and earlier, Apple Clang 11.0.0 and earlier, and CUDA 10.2 and earlier (#333, #334)

Fixes:

• Use NVCC's __builtin_assume() for gsl_ASSUME_() in device code (#324, thanks to @codecircuit)
• Fix not_null_ic<> copy construction (#326, thanks to @runer112)
• Respect libc++ configuration option to disable wchar_t (#331, thanks to @burnpanck)
• Suppress "-Wweak-vtables" warning for Clang (#332, thanks to @mrahn)
• Suppress "-Wuseless-cast" warning for GCC (#332, thanks to @wirew0rm)