The FEN validation check intended to reject pawns on the first or eighth rank uses the `Rank` enum values in a bitwise OR operation:
`if (pieces(PAWN) & (RANK_1 | RANK_8))`
`RANK_1 | RANK_8` evaluates to the integer `0 | 7 == 7` instead of a bitboard, so the expression only tests squares A1, B1 and C1. As a result, unsupported positions with pawns elsewhere on the first or eighth rank are silently accepted. For instance, `position fen 3P3k/8/8/8/8/8/8/3K4 w - - 0 1` is accepted even though the pawn on d8 makes the position unsupported.
Use the `Rank1BB | Rank8BB` bitboard constants so any pawn on the first or eighth rank are correctly rejected.
closes https://github.com/official-stockfish/Stockfish/pull/6887
No functional change
