spring.grpc.server.health.include-overall-health is not taken into account

[xfocus3]

Fixes gh-50791

spring.grpc.server.health.include-overall-health=false was bound but not used by the auto-configured gRPC health components. The overall server component was always created, so scheduled gRPC health updates still evaluated all health contributors even when overall health reporting was disabled.

This change makes the auto-configured server component nullable and omits it when overall health reporting is disabled. Service-specific health components continue to be created from spring.grpc.server.health.service.* settings.

Verification:

• RED: ./gradlew :module:spring-boot-grpc-server:test --tests org.springframework.boot.grpc.server.autoconfigure.health.AutoConfiguredHealthCheckedGrpcComponentsTests.getServerWhenIncludeOverallHealthIsFalseReturnsNull --no-daemon failed before the fix.
• GREEN: ./gradlew :module:spring-boot-grpc-server:test --tests org.springframework.boot.grpc.server.autoconfigure.health.AutoConfiguredHealthCheckedGrpcComponentsTests.getServerWhenIncludeOverallHealthIsFalseReturnsNull --tests org.springframework.boot.grpc.server.autoconfigure.health.GrpcServerHealthAutoConfigurationTests.whenIncludeOverallHealthIsFalseDoesNotReportOverallHealth --no-daemon
• GREEN: ./gradlew :module:spring-boot-grpc-server:test --tests org.springframework.boot.grpc.server.autoconfigure.health.AutoConfiguredHealthCheckedGrpcComponentsTests --tests org.springframework.boot.grpc.server.autoconfigure.health.GrpcServerHealthAutoConfigurationTests --no-daemon
• GREEN: ./gradlew :module:spring-boot-grpc-server:checkstyleMain :module:spring-boot-grpc-server:checkstyleTest --no-daemon
• GREEN: git diff --check HEAD^ HEAD

[snicoll]

Thanks @xfocus3 ! I am not sure it completely fixes the related issue so I've left it open.