Replace @OptionalParameter with JSpecify's @Nullable

[philwebb]

See #45389

Before 4.0.0 GA we should consider if such an annotation is necessary or if we should use JSpecify annotations instead.

[snicoll]

We've discussed this and decided to keep a dedicated annotation for this as we believe that JSpecify should be optional.

[wilkinsona]

On further thought, we'd decided to do this.

With JSpecify being a required dependency of spring-core, @Nullable will be available to anyone writing an Actuator endpoint. As such, it will be just as easy to use @Nullable as it would have been to use @OptionalParameter. It also eases the upgrade experience slightly as it's just a change in package name to switch from Framework's deprecated @Nullable to JSpecify's. Under the covers, the implementation can use Framework's Nullness.forMethodParameter(MethodParameter) and then also benefit from Kotlin null-safety support.