Remove TODO in a test for verifying start of the week (#50894)

The test should be run against 7.7 version at least, as this was only backported and released in that version

relates SPI based implementation #48209
relates backport #50916

Author: pgomulka

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/310_date_agg_per_day_of_week.yml

@@ -1,8 +1,8 @@
 ---
 setup:
   - skip:
-      version: " - 7.99.99" #TODO change this after backport
-      reason:  "not backported yet"
+      version: " - 7.6.99"
+      reason:  "Start of the week Monday was enabled in a backport to 7.7 PR#50916"
 
   - do:
         indices.create: