Fix MonotonicRecordTimeIT for pruned participants (#11423)

Reading explicitly from ledger.begin causes the test to fail on a pruned
participant. The test framework automatically selects a ledger offset
observed at the start of the test anyway, so there's no need to
explicitly put ledger.begin as the starting offset for the checkpoints.

CHANGELOG_BEGIN
CHANGELOG_END

ledger/ledger-api-test-tool/src/main/scala/com/daml/ledger/api/testtool/suites/MonotonicRecordTimeIT.scala

@@ -24,7 +24,7 @@ final class MonotonicRecordTimeIT extends LedgerTestSuite {
       _ <- Future.traverse(1 to Submissions) { _ =>
         ledger.create(party, Dummy(party))
       }
-      checkpoints <- ledger.checkpoints(Submissions, ledger.begin)(party)
+      checkpoints <- ledger.checkpoints(Submissions)(party)
     } yield {
       assertLength(s"At least $Submissions checkpoints available", Submissions, checkpoints)