Align PostgreSQL Testcontainers image version across integration tests

[seonwooj0810]

Summary

Aligns the PostgreSQL Testcontainers image version across the four PostgreSQL-based integration tests in spring-batch. After this change all four tests use postgres:18.1.

File	Before	After
spring-batch-core/.../migration/PostgreSQLMigrationScriptIntegrationTests.java	postgres:18.1	postgres:18.1 (unchanged)
spring-batch-core/.../PostgreSQLJobRepositoryIntegrationTests.java	postgres:17.5	postgres:18.1
spring-batch-infrastructure/.../PostgresPagingQueryProviderIntegrationTests.java	postgres:17.5	postgres:18.1
spring-batch-samples/.../PostgreSQLJpaIntegrationTests.java	postgres:17.5	postgres:18.1

This eliminates the drift described in #5383, where the migration tests and the rest of the suite were validated against different PostgreSQL major versions.

Test plan

• Confirmed before the change: three files referenced postgres:17.5, one referenced postgres:18.1.
• Confirmed after the change: all four files reference postgres:18.1.
• Diff is a literal version-string update inside DockerImageName.parse(...) — no logic change.
• Full ./mvnw verify could not be executed locally: the spring-batch-infrastructure main compile fails on JDK 21 with a pre-existing NullAway / JSpecify error (Running NullAway in JSpecify mode requires either JDK 22+ ...). The project's CI uses Java 25 (.github/workflows/continuous-integration.yml), so this is an environment limitation, not a regression introduced by this change. Verified by reproducing the same error against unmodified main.

Verification done

1. No in-flight PR: gh pr list --repo spring-projects/spring-batch --search "postgres testcontainers" / "5383" / "postgres:18.1" returned no matching PR; /issues/5383/events shows only the original labeled event with no cross-reference.
2. No active claim: the issue author mentioned "I would like to submit a PR for this" 33 days ago (2026-04-21) without follow-up, so the implicit claim is treated as stalled.
3. Code-focused: change touches three .java integration test files.
4. Verified the bug still exists on main via gh search code: 3 files use postgres:17.5, 1 uses postgres:18.1 (matching the issue body exactly).
5. No umbrella/parent issue closed silently — issue is still open with status: waiting-for-triage.

Resolves #5383