scripts/retire: fix "created on reception" epoch

The epoch was set to when the sync PR's commit was committed, not when
the merge commit merging the PR was committed.

Since we are using `--first-parent`, we need the latter.

Author: MattSturgeon

scripts/retire.sh

@@ -60,7 +60,7 @@ newCutoff=$(date --date="1 year ago" +%s)
 # For now however, the code needs to check if the file creation date
 # is before 2025-07-09 to distinguish between periods A and C,
 # so we hardcode that date for the code to use.
-createdOnReceptionEpoch=$(date --date=2025-07-09 +%s)
+createdOnReceptionEpoch=$(date --date=2025-07-17 +%s)
 
 if [[ -z "${PROD:-}" ]]; then
   tmp=$(git rev-parse --show-toplevel)/.tmp