fix: downgrade to Java 17 for CI build compatibility

Build container has JDK 17, not 21. Also replaced
Thread.ofVirtual() (Java 21) with standard Thread.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -1,4 +1,4 @@
-FROM --platform=$BUILDPLATFORM maven:3.9-eclipse-temurin-21 AS backend-build
+FROM --platform=$BUILDPLATFORM maven:3.9-eclipse-temurin-17 AS backend-build
 WORKDIR /build
 COPY pom.xml .
 RUN mvn dependency:go-offline -B || true
@@ -14,7 +14,7 @@ RUN echo "//gitea.siegeln.net/api/packages/cameleer/npm/:_authToken=${REGISTRY_T
 COPY ui/ .
 RUN npm run build
 
-FROM eclipse-temurin:21-jre
+FROM eclipse-temurin:17-jre
 WORKDIR /app
 COPY --from=backend-build /build/target/cameleer-deploy-demo-*.jar /app/server.jar
 COPY --from=ui-build /ui/dist /app/static