chore(xtask): tweak the TWiM report to include only merged PRs, not created PRs

As an outsider, I am mostly interested in features and new developments
that have happened, not those that *may* happen. An open-but-not-merged
PR may not get merged in the end, or it may not get merged any time
soon, creating false expectations. Merged PRs, on the other hand, have
definitely happened (even if they get undone, that happens via other PRs
that will get merged later). As such, I think it brings more value to
outsiders.

xtask/src/release.rs

@@ -142,7 +142,7 @@ fn weekly_report() -> Result<()> {
 
     cmd!(
         sh,
-        "gh pr list --search created:>{one_week_ago} --json {JSON_FIELDS} --template {template}"
+        "gh pr list --search merged:>{one_week_ago} --json {JSON_FIELDS} --template {template}"
     )
     .quiet()
     .run()?;