Introduce JSpecify annotations for nullability constraints

[scordio]

Following spring-projects/spring-framework#28797 and spring-projects/spring-batch#4673, all the extension modules currently on the latest Spring Batch version could adopt JSpecify annotations to ensure build-time validation via NullAway.

See also:

• https://spring.io/blog/2025/03/10/null-safety-in-spring-apps-with-jspecify-and-null-away
• https://docs.openrewrite.org/recipes/java/jspecify/migratefromspringframeworkannotations

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• [ ] spring-batch-bigquery
• [ ] spring-batch-elasticsearch
• [ ] spring-batch-excel
• [ ] spring-batch-neo4j
• [x] spring-batch-notion (#191)