Restore JsonLineMapper compatibility with Jackson 2

[answndud]

Fixes #5330

JsonLineMapper currently instantiates Jackson 3's JsonMapper by default, which makes the default constructor unusable when only Jackson 2 is present on the classpath.

This change updates JsonLineMapper to:

• keep the existing Jackson 3 constructor
• add a Jackson 2 ObjectMapper constructor
• fall back to Jackson 2 when Jackson 3 is not available
• add regression tests for both the explicit Jackson 2 constructor and the default-constructor fallback path

Tests:

• ./mvnw -pl spring-batch-infrastructure -Dtest=org.springframework.batch.infrastructure.item.file.mapping.JsonLineMapperTests test

[quaff]

I don't think it's a good idea to mix usage of two version of Jackson, it's better to write your own JsonLineMapper if you insist on using Jackson2 for Spring Batch 6.