Skip PRRL renewal on UNASSIGNED_SEQ_NO

Today when renewing PRRLs we assert that any invalid "backwards" renewals must
be because we are recovering the shard. In fact it's also possible to have
`checkpointState.globalCheckpoint == SequenceNumbers.UNASSIGNED_SEQ_NO` on a
tracked shard copy if the primary was just promoted and hasn't received
checkpoints from all of its peers too.

This commit weakens the assertion to match.

Caught by a [failure of the full cluster restart
tests](https://scans.gradle.com/s/5lllzgqtuegty/console-log#L8605)

Relates elastic#41536

server/src/main/java/org/elasticsearch/index/seqno/ReplicationTracker.java

@@ -508,10 +508,9 @@ public synchronized void renewPeerRecoveryRetentionLeases() {
                                 PEER_RECOVERY_RETENTION_LEASE_SOURCE);
                         } else {
                             // the retention lease is tied to the node, not the shard copy, so it's possible a copy was removed and now
-                            // we are in the process of recovering it again. The recovery process will fix the lease before initiating
-                            // tracking on this copy:
-                            assert checkpointState.tracked == false
-                                && checkpointState.globalCheckpoint == SequenceNumbers.UNASSIGNED_SEQ_NO :
+                            // we are in the process of recovering it again, or maybe we were just promoted and have not yet received the
+                            // global checkpoints from our peers.
+                            assert checkpointState.globalCheckpoint == SequenceNumbers.UNASSIGNED_SEQ_NO :
                                 "cannot renew " + retentionLease + " according to " + checkpointState + " for " + shardRouting;
                         }
                     }