Set retention to 25 months on firefox_desktop_derived.pageload_1pct_v1 (#6513)

sql/moz-fx-data-shared-prod/firefox_desktop_derived/pageload_1pct_v1/metadata.yaml

@@ -18,6 +18,7 @@ bigquery:
     require_partition_filter: true
     expiration_days: null
   range_partitioning: null
+  expiration_days: 780
   clustering:
     fields:
     - normalized_channel