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

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