Fix GitHub Actions build by aligning Java runtime and decoupling tests from local PostgreSQL

[Copilot]

The build job was failing because CI compiled with JDK 17 while the project targets Java 21. After correcting that mismatch, test execution also depended on a local PostgreSQL instance, which is unavailable in Actions.

• CI runtime alignment

  • Updated .github/workflows/maven.yml to use Java 21 in actions/setup-java, matching pom.xml (<java.version>21</java.version>).

• Test isolation from external DB

  • Added com.h2database:h2 as a test-scoped dependency.
  • Added src/test/resources/application.properties with in-memory H2 datasource settings (PostgreSQL compatibility mode) so Spring context tests do not require localhost:5432.

• Context test configuration cleanup

  • Kept StegoshieldGatewayApplicationTests as a standard @SpringBootTest, relying on test profile datasource config instead of disabling JDBC/JPA autoconfiguration.

# .github/workflows/maven.yml
- name: Set up JDK 21
  uses: actions/setup-java@v4
  with:
    java-version: '21'
    distribution: 'temurin'
    cache: maven

[sayednowshad]

ok