Simplify relative_age and its constants
STC: https://tests.stockfishchess.org/tests/view/69cf29ed1668971c9da23a5a
LLR: 3.47 (-2.94,2.94) <-1.75,0.25>
Total: 193760 W: 50142 L: 50051 D: 93567
Ptnml(0-2): 534, 21375, 52980, 21448, 543
* Simplify retval and arithmetic semantics
* Remove mixing of int and uint8_t
I find this overall much easier to reason about
(Also rename a DEPTH constant per mstembera's suggestion https://github.com/official-stockfish/Stockfish/pull/5766#issuecomment-2586449296)
closes https://github.com/official-stockfish/Stockfish/pull/6702
No functional change