Migrate CompositeHealth/HealthComponent to Spring Boot 4 descriptor types

[steve-aom-elliott]

What's missing

In Spring Boot 4 the actuator health response API was reorganised:

Spring Boot 3	Spring Boot 4
org.springframework.boot.actuate.health.HealthComponent (abstract)	org.springframework.boot.health.actuate.endpoint.HealthDescriptor (abstract)
org.springframework.boot.actuate.health.CompositeHealth	org.springframework.boot.health.actuate.endpoint.CompositeHealthDescriptor
HealthEndpoint.health() returns HealthComponent	HealthEndpoint.health() returns HealthDescriptor

MigrateToModularStarters already handles every other org.springframework.boot.actuate.health.* type explicitly and then falls through to a catch-all ChangePackage that maps the remainder to org.springframework.boot.health.contributor. That catch-all sent CompositeHealth and HealthComponent to the wrong package and didn't rename the classes, so callers like the one below still failed to compile under SB4:

HealthComponent health = healthEndpoint.health();
if (health instanceof CompositeHealth composite) {
    body.put("components", composite.getComponents());
}

Change

Add two ChangeType entries before the catch-all ChangePackage, alongside the other types that move to health.actuate.endpoint:

• org.springframework.boot.actuate.health.CompositeHealth → org.springframework.boot.health.actuate.endpoint.CompositeHealthDescriptor
• org.springframework.boot.actuate.health.HealthComponent → org.springframework.boot.health.actuate.endpoint.HealthDescriptor

getComponents() exists on CompositeHealthDescriptor with the same signature shape (returning Map<String, HealthDescriptor>), so the rest of the call site doesn't need touching.

org.springframework.boot.actuate.health.Health is unchanged — it still ends up at org.springframework.boot.health.contributor.Health via the catch-all (it's the indicator output type, not a HealthDescriptor).

Test plan

• New test MigrateActuateHealthPackage#migrateCompositeHealthAndHealthComponentToDescriptors exercises the instanceof + getComponents() pattern from a real controller
• All tests in MigrateToModularStartersTest pass locally