Migrate multiple constexpr-s to C++20 syntax

[simonmicro]

• Ports template-chains to if ... ( require {} )
• Switched to auto concepts
• Fixed some more String-class bugs :fire:

:warning: THIS PR IS NOT WORKING YET; THE GCC-VERSION IS TOO OLD / HAS A BUG, WHICH PREVENTS THIS FROM COMPILING ON THE TARGET PLATFORM! The emulator works regardless. :grinning:

Reference of upstream progress:

• [x] ESP-IDF: https://github.com/espressif/crosstool-NG/issues/36 -> fixed in 5.0+
• [ ] Arduino-ESP Framework has to use / work with ESP-IDF 5.0+: https://github.com/espressif/arduino-esp32 // https://docs.espressif.com/projects/arduino-esp32/en/latest/esp-idf_component.html // https://github.com/orgs/espressif/projects/3-> waiting
• [ ] Update this projects platformio.ini to use the new Arduino-ESP Framework