[CI] SearchHitTests.testSerializeShardTarget fails on 6.x

[tlrx]

After #36997, the SearchHitTests.testSerializeShardTarget needs to be adapted to work on 6.x when a random version < 5.6 is picked up and the cluster alias is not deserialized.

See:
https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+6.x+release-tests/317/consoleFull

I can reproduce on 6.x with:

./gradlew :server:unitTest -Dtests.seed=6E493E03085BDF45 -Dtests.class=org.elasticsearch.search.SearchHitTests -Dtests.method="testSerializeShardTarget" -Dtests.security.manager=true -Dbuild.snapshot=true -Dtests.jvm.argline="-Dbuild.snapshot=true" -Dtests.locale=sr-BA -Dtests.timezone=Australia/ACT -Dcompiler.java=11 -Druntime.java=8

[elasticmachine]

Pinging @elastic/es-search

[tlrx]

Test muted on 6.x in 3bca3d4

[javanna]

Fixed in 890fa34