Adjust skip version for shrink index test

This commit adjusts the skip version for a shrink index test that
ensures that a shrunken index ignores templates; the version can be
adjusted after the fix was backported targeting 5.6.0 and later.

Relates #25380

rest-api-spec/src/main/resources/rest-api-spec/test/indices.shrink/20_source_mapping.yml

@@ -1,7 +1,7 @@
 ---
 "Shrink index ignores target template mapping":
     - skip:
-        version: " - 5.99.99"
+        version: " - 5.5.99"
         reason:  bug fixed in 6.0
 
     - do: