TEST: loosen translog uncommitted size rest test

If a primary executing the only indexing request is on a 6.2 node, the
translog uncommitted size will equal to the translog creation size
because there is no overhead (eg. multiple empty translog generations)
on the old node. Moreover, since other primaries on current nodes did
not receive writes, the flush request is no-op on these shards, translog
uncommitted size does not change. In this case, the total translog
uncommitted size on all primaries equal to the translog creation size.

This commit loosens the assertion from less-than to less-than-or-equal
to cover this case.

Relates #28676

rest-api-spec/src/main/resources/rest-api-spec/test/indices.stats/20_translog.yml

@@ -47,7 +47,7 @@ setup:
   - gt: { indices.test.primaries.translog.size_in_bytes: $creation_size }
   - match: { indices.test.primaries.translog.operations: 1 }
   ## creation translog size has some overhead due to an initial empty generation that will be trimmed later
-  - lt: { indices.test.primaries.translog.uncommitted_size_in_bytes: $creation_size }
+  - lte: { indices.test.primaries.translog.uncommitted_size_in_bytes: $creation_size }
   - match: { indices.test.primaries.translog.uncommitted_operations: 0 }
 
   - do: