Increase MKS MINI 12864 default contrast (#15139)

Marlin/src/inc/Conditionals_post.h

   #define _LCD_CONTRAST_MIN  120
   #define _LCD_CONTRAST_INIT 190
 #elif ENABLED(MKS_MINI_12864)
-  #define _LCD_CONTRAST_INIT 150
+  #define _LCD_CONTRAST_MIN  120
+  #define _LCD_CONTRAST_INIT 195
 #elif ANY(FYSETC_MINI_12864_X_X, FYSETC_MINI_12864_1_2, FYSETC_MINI_12864_2_0, FYSETC_MINI_12864_2_1)
   #define _LCD_CONTRAST_INIT 220
 #elif ENABLED(ULTI_CONTROLLER)