btrfs-progs: docs: add 6.12 kernel development statistics

Update graphs.

[ci skip]

Signed-off-by: David Sterba <[email protected]>

Author: kdave

Documentation/Contributors.rst

@@ -56,6 +56,7 @@ Statistics for 6.x series
     "6.9", "19", "110727", "161231", "147", "+2476 -1323"
     "6.10", "21", "110878", "161751", "154", "+2993 -2473"
     "6.11", "18", "111848", "163484", "188", "+5776 -4043"
+    "6.12", "20", "111881", "163548", "148", "+1868 -1804"
 
 
 Legend: