backupccl: enable restore_span.target_size

This setting was previously disabled because of timeouts being
observed when restoring our TPCCInc fixtures. The cause of those
timeouts has been identified as
cockroachdb#88329 making it safe
to re-enable merging of spans during restore. This settings prevents
restore from over-splitting and leaving the cluster with a merge hangover
post restore.

Informs: cockroachdb#86470

Release note (sql change): Sets `backup.restore_span.target_size`
to default to 384 MiB so that restore merges upto that size of spans
when reading from the backup before actually ingesting data. This should
reduce the number of ranges created during restore and thereby reduce
the merging of ranges that needs to occur post restore.

pkg/ccl/backupccl/restore_span_covering.go

@@ -50,7 +50,7 @@ var targetRestoreSpanSize = settings.RegisterByteSizeSetting(
 	settings.TenantWritable,
 	"backup.restore_span.target_size",
 	"target size to which base spans of a restore are merged to produce a restore span (0 disables)",
-	0, //TODO(dt): make this something like 384 << 20,
+	384<<20,
 )
 
 // makeSimpleImportSpans partitions the spans of requiredSpans into a covering