Switch startup actuator endpoint to using HTTP POST

Closes gh-23213

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/endpoint/web/documentation/StartupEndpointDocumentationTests.java

@@ -32,7 +32,7 @@ import org.springframework.restdocs.payload.ResponseFieldsSnippet;
 import static org.springframework.restdocs.mockmvc.MockMvcRestDocumentation.document;
 import static org.springframework.restdocs.payload.PayloadDocumentation.fieldWithPath;
 import static org.springframework.restdocs.payload.PayloadDocumentation.responseFields;
-import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.post;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
 
 /**
@@ -75,7 +75,7 @@ class StartupEndpointDocumentationTests extends MockMvcEndpointDocumentationTest
 				fieldWithPath("timeline.events.[].startupStep.tags[].value")
 						.description("The value of the StartupStep Tag."));
 
-		this.mockMvc.perform(get("/actuator/startup")).andExpect(status().isOk())
+		this.mockMvc.perform(post("/actuator/startup")).andExpect(status().isOk())
 				.andDo(document("startup", responseFields));
 	}
 

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/startup/StartupEndpoint.java

@@ -18,7 +18,7 @@ package org.springframework.boot.actuate.startup;
 
 import org.springframework.boot.SpringBootVersion;
 import org.springframework.boot.actuate.endpoint.annotation.Endpoint;
-import org.springframework.boot.actuate.endpoint.annotation.ReadOperation;
+import org.springframework.boot.actuate.endpoint.annotation.WriteOperation;
 import org.springframework.boot.context.metrics.buffering.BufferingApplicationStartup;
 import org.springframework.boot.context.metrics.buffering.StartupTimeline;
 
@@ -44,7 +44,7 @@ public class StartupEndpoint {
 		this.applicationStartup = applicationStartup;
 	}
 
-	@ReadOperation
+	@WriteOperation
 	public StartupResponse startup() {
 		StartupTimeline startupTimeline = this.applicationStartup.drainBufferedTimeline();
 		return new StartupResponse(startupTimeline);