Decrease the "recent spend window" in gamma re-select to 15 blocks

- combined with patching integer truncation (#7798), this gets the algorithm marginally closer to mirroring empirically observed output ages
- 50 was originally chosen assuming integer truncation would remain in the client for that client release version. But patching integer truncation causes the client to select more outputs in the 10-100 block range, and therefore the benefit of choosing a larger recent spend window of 50 has less merit
- 15 seems well-suited to cover the somewhat sizable observable gap in the early window of blocks

src/wallet/wallet2.cpp

@@ -143,7 +143,7 @@ using namespace cryptonote;
 #define IGNORE_LONG_PAYMENT_ID_FROM_BLOCK_VERSION 12
 
 #define DEFAULT_UNLOCK_TIME (CRYPTONOTE_DEFAULT_TX_SPENDABLE_AGE * DIFFICULTY_TARGET_V2)
-#define RECENT_SPEND_WINDOW (50 * DIFFICULTY_TARGET_V2)
+#define RECENT_SPEND_WINDOW (15 * DIFFICULTY_TARGET_V2)
 
 static const std::string MULTISIG_SIGNATURE_MAGIC = "SigMultisigPkV1";
 static const std::string MULTISIG_EXTRA_INFO_MAGIC = "MultisigxV1";