This change removes unused definitions from the CMake files, moves variable definitions from `XrplSanity` to `XrplSettings` where they better belong, and updates the minimum GCC and Clang versions to match what we actually minimally support.
Per XLS-0095, we are taking steps to rename ripple(d) to xrpl(d).
This change updates the CMake files and definitions therein, plus a handful of related modifications. Specifically, the compiler files are renamed from `RippleXXX.cmake` or `RippledXXX.cmake` to `XrplXXX.cmake`, and any references to `ripple` and `rippled` (with or without capital letters) are renamed to `xrpl` and `xrpld`, respectively. The name of the binary, currently `rippled`, remains unchanged and will be updated in a separate PR. This change is purely cosmetic and does not affect the functioning of the binary.
