Fix index version constant

[jordan-powers]

We need to use a patch version, not a full version here. 9.1.8 was the previous commit (50f58de), so this hasn't been released yet - we can modify this in time for 9.1.9.

This is the same problem as fixed in #139361 on 9.2.

Discovered by #139482.

[elasticsearchmachine]

Pinging @elastic/es-security (Team:Security)

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[martijnvg]

LGTM

[jfreden]

Some upgrade tests are failing with:

org.elasticsearch.transport.RemoteTransportException: [test-cluster-2][127.0.0.1:34763][internal:cluster/coordination/join]
Caused by: java.lang.IllegalStateException: index [.inference/b-HfCvLdRY27B9W3DI4Zpw] version not supported: 9.1.9 maximum compatible index version is: 9.1.9

[jfreden]

One of the tests that fail is:

./gradlew ":x-pack:plugin:inference:qa:rolling-upgrade:bcUpgradeTest" -Dtests.class="org.elasticsearch.xpack.application.OpenAiServiceUpgradeIT" -Dtests.method="testOpenAiEmbeddings {upgradedNodes=1}" -Dtests.seed=A7F17D71A1B794CB -Dtests.bwc.main.version=9.1.9-SNAPSHOT -Dtests.bwc.refspec.main=c6ce60656077305524f7950a6b01f0d442d3f575 -Dtests.locale=ar-Arab-EG -Dtests.timezone=Etc/GMT+2 -Druntime.java=25

So it's doing an upgrade from 9.1.0-SNAPSHOT (current 9.1 "main", see this), to the version in the PR, but the index version in the PR code is older than on 9.1, because the fix in this PR is not merged yet. Therefore this error is expected and can be ignored.