Fix ee_min and max Value() to use image$reduceRegion()

[Wycology]

This PR fixes ee_minValue() and ee_maxValue() which broke after recent rgee updates.
The function now calls reduceRegion() as a method on the image object instead of passing image= as an argument.
This is compatible with current rgee versions and Earth Engine API.
I tested the function on a MODIS NDVI image clipped to Kenya and it returns the expected minimum value.

devtools::load_all()
library(rgee)
ee_Initialize()

roi <- ee$Geometry$Rectangle(
  35.35, -0.36,
  35.36, -0.35
)

ndvi <- ee$ImageCollection("MODIS/061/MOD13Q1")$
  filterDate("2025-10-01", "2025-11-01")$
  select("NDVI")$
  mean()$
  multiply(0.0001)$
  clip(roi)

ee_maxValue(ndvi)