Align PostgreSQL Testcontainers image version across integration tests

[config25]

Problem

The PostgreSQL image version used in Testcontainers-based integration tests has
drifted between files. Currently:

• postgres:18.1 is used in:

  • spring-batch-core/.../migration/PostgreSQLMigrationScriptIntegrationTests.java

• postgres:17.5 is used in:

  • spring-batch-core/.../PostgreSQLJobRepositoryIntegrationTests.java
  • spring-batch-infrastructure/.../PostgresPagingQueryProviderIntegrationTests.java
  • spring-batch-samples/.../PostgreSQLJpaIntegrationTests.java

The drift was introduced in commit 0d1b5b103 ("Refine contribution #4954"),
which updated only the migration tests to postgres:18.1 while the other
three were left on postgres:17.5 from the earlier bulk upgrade in
commit 584c3c6ea (PR #4889, June 2025).

As a result, the migration scripts and the rest of the suite are validated
against different PostgreSQL major versions.

Proposed fix

Align all four files to postgres:18.1, matching the latest maintainer-chosen
version.

I would like to submit a PR for this.