Per [XLS-0095](https://xls.xrpl.org/xls/XLS-0095-rename-rippled-to-xrpld.html), we are taking steps to rename ripple(d) to xrpl(d).
This change modifies the binary name from `rippled` to `xrpld`, and creates a symlink named `rippled` that points to the `xrpld` binary.
Note that https://github.com/XRPLF/rippled/pull/5975 renamed any references to `rippled` in the CMake files and their contents, but explicitly maintained the `rippled` binary name by adding an exception. This change now undoes this exception and adds an explicit symlink instead.
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.
