Align `@Nullable` annotation placement with Spring code style

[acktsap]

Spring Batch uses JSpecify annotations for null safety.

Some @Nullable annotations are not placed immediately before the annotated type, as recommended by the Spring Framework reference documentation.

Examples from the current main branch:

• @Nullable public static Duration
• private @Nullable final String
• @Nullable default JobInstance

The recommended placement is:

public static @Nullable Duration calculateDuration(...);

private final @Nullable String message;

default @Nullable JobInstance getJobInstance(...);

see also: https://github.com/spring-projects/spring-framework/wiki/Code-Style#null-safety-of-apis-and-fields