Add --warning-mode none when assembling OpenSearch in CI (#231)

* Add `--warning-mode none` when assembling OpenSearch in CI

This should hopefully fix the integration test with unreleased OpenSearch
versions, as per:

#229 (review)

Signed-off-by: Tim Düsterhus <tim@tideways-gmbh.com>

* Build OpenSearch with Temurin 21

The OpenJDK installed by default appears to be:

> JDK Version           : 11 (Eclipse Temurin JDK)

which apparently is too old.

Signed-off-by: Tim Düsterhus <tim@tideways-gmbh.com>

---------

Signed-off-by: Tim Düsterhus <tim@tideways-gmbh.com>

.github/workflows/test_unreleased.yml

@@ -50,10 +50,16 @@ jobs:
           path: opensearch/distribution/archives/linux-tar/build/distributions
           key: ${{ steps.get-key.outputs.key }}
 
+      - uses: actions/setup-java@v4
+        if: steps.cache-restore.outputs.cache-hit != 'true'
+        with:
+          distribution: "temurin"
+          java-version: "21"
+
       - name: Assemble OpenSearch
         if: steps.cache-restore.outputs.cache-hit != 'true'
         working-directory: opensearch
-        run: ./gradlew :distribution:archives:linux-tar:assemble
+        run: ./gradlew :distribution:archives:linux-tar:assemble --warning-mode none
 
       - name: Save cached build
         if: steps.cache-restore.outputs.cache-hit != 'true'