fix: Add missing import for Assert in SslHealthIndicator

The previous commit introduced an `Assert.notNull` check in the
SslHealthIndicator constructor but missed the required import for
`org.springframework.util.Assert`. This caused the pull request
build to fail.

This commit adds the necessary import statement to resolve the
compilation error.

Signed-off-by: geniuus <[email protected]>

Author: geniusYoo

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/ssl/SslHealthIndicator.java

@@ -28,6 +28,7 @@
 import org.springframework.boot.info.SslInfo.BundleInfo;
 import org.springframework.boot.info.SslInfo.CertificateChainInfo;
 import org.springframework.boot.info.SslInfo.CertificateInfo;
+import org.springframework.util.Assert;
 
 /**
  * {@link HealthIndicator} that checks the certificates the application uses and reports