Use a single --initialize-at-build-time parameter per file

Closes gh-28563

Author: sdeleuze

spring-core/src/main/resources/META-INF/native-image/org.springframework/spring-core/native-image.properties

@@ -1,2 +1,2 @@
-Args = --initialize-at-build-time=org.springframework.aot.AotDetector \
---initialize-at-build-time=org.springframework.core.NativeDetector
+Args = --initialize-at-build-time=org.springframework.aot.AotDetector,\
+org.springframework.core.NativeDetector