Update t-shirt script for 6 month graphs

The 3 month graphs were removed in #25175.

task-9889/tshirt.py

@@ -88,13 +88,13 @@ def calculate_2mo_avg(response, response_type):
 
   # Check if required data is present in the response
   if response_type == 'uptime':
-    if '3_months' not in response['uptime']:
+    if '6_months' not in response['uptime']:
       return -1
-    data = response['uptime']['3_months']
+    data = response['uptime']['6_months']
   elif response_type == 'bandwidth':
-    if '3_months' not in response['write_history']:
+    if '6_months' not in response['write_history']:
       return -1
-    data = response['write_history']['3_months']
+    data = response['write_history']['6_months']
   # Sum up all values within past 2 months
   _sum = 0
   count = 0