Add gcc, clang, apple-clang support for C++26

[AbrilRBS]

Changelog: Feature: Add C++26 support for gcc, clang, and apple-clang.
Changelog: Fix: Set C++20 flag to {gnu}c++20 for gcc >= 10 instead of c++2a until gcc 12.
Changelog: Fix: Set C++23 flag to {gnu}c++23 for gcc >= 11 instead of c++2b.
Docs: conan-io/docs#3878

We're accepting the same risks gcc/clang do when supporting the c++26 flags out of the gate, let's hope it pans out 🤞

gcc/clang versions based on https://godbolt.org/z/K3KPqrhWW
gcc c++20/c++23 flag fix based on https://godbolt.org/z/bKh6djM7E
apple-clang support based on:

• apple-clang 16:

note: use 'c++98' or 'c++03' for 'ISO C++ 1998 with amendments' standard
note: use 'gnu++98' or 'gnu++03' for 'ISO C++ 1998 with amendments and GNU extensions' standard
note: use 'c++11' for 'ISO C++ 2011 with amendments' standard
note: use 'gnu++11' for 'ISO C++ 2011 with amendments and GNU extensions' standard
note: use 'c++14' for 'ISO C++ 2014 with amendments' standard
note: use 'gnu++14' for 'ISO C++ 2014 with amendments and GNU extensions' standard
note: use 'c++17' for 'ISO C++ 2017 with amendments' standard
note: use 'gnu++17' for 'ISO C++ 2017 with amendments and GNU extensions' standard
note: use 'c++20' for 'ISO C++ 2020 DIS' standard
note: use 'gnu++20' for 'ISO C++ 2020 DIS with GNU extensions' standard
note: use 'c++23' for 'ISO C++ 2023 DIS' standard
note: use 'gnu++23' for 'ISO C++ 2023 DIS with GNU extensions' standard
note: use 'c++2c' or 'c++26' for 'Working draft for C++2c' standard
note: use 'gnu++2c' or 'gnu++26' for 'Working draft for C++2c with GNU extensions' standard

• apple-clang 15:

note: use 'c++98' or 'c++03' for 'ISO C++ 1998 with amendments' standard
note: use 'gnu++98' or 'gnu++03' for 'ISO C++ 1998 with amendments and GNU extensions' standard
note: use 'c++11' for 'ISO C++ 2011 with amendments' standard
note: use 'gnu++11' for 'ISO C++ 2011 with amendments and GNU extensions' standard
note: use 'c++14' for 'ISO C++ 2014 with amendments' standard
note: use 'gnu++14' for 'ISO C++ 2014 with amendments and GNU extensions' standard
note: use 'c++17' for 'ISO C++ 2017 with amendments' standard
note: use 'gnu++17' for 'ISO C++ 2017 with amendments and GNU extensions' standard
note: use 'c++20' for 'ISO C++ 2020 DIS' standard
note: use 'gnu++20' for 'ISO C++ 2020 DIS with GNU extensions' standard
note: use 'c++2b' for 'Working draft for ISO C++ 2023 DIS' standard
note: use 'gnu++2b' for 'Working draft for ISO C++ 2023 DIS with GNU extensions' standard

Closes #17073