Adapt to deprecation of rejectIllegalHeader

Closes gh-35442

Author: wilkinsona

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ServerProperties.java

@@ -459,6 +459,7 @@ public static class Tomcat {
 		/**
 		 * Whether to reject requests with illegal header names or values.
 		 */
+		@Deprecated
 		private boolean rejectIllegalHeader = true;
 
 		/**
@@ -612,6 +613,7 @@ public void setConnectionTimeout(Duration connectionTimeout) {
 			this.connectionTimeout = connectionTimeout;
 		}
 
+		@DeprecatedConfigurationProperty(reason = "The setting has been deprecated in Tomcat")
 		public boolean isRejectIllegalHeader() {
 			return this.rejectIllegalHeader;
 		}

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/embedded/TomcatWebServerFactoryCustomizer.java

@@ -220,6 +220,7 @@ private void customizeRelaxedQueryChars(ConfigurableTomcatWebServerFactory facto
 		factory.addConnectorCustomizers((connector) -> connector.setProperty("relaxedQueryChars", relaxedChars));
 	}
 
+	@SuppressWarnings("deprecation")
 	private void customizeRejectIllegalHeader(ConfigurableTomcatWebServerFactory factory, boolean rejectIllegalHeader) {
 		factory.addConnectorCustomizers((connector) -> {
 			ProtocolHandler handler = connector.getProtocolHandler();

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/embedded/TomcatWebServerFactoryCustomizerTests.java

@@ -365,6 +365,8 @@ void disableRemoteIpValve() {
 	}
 
 	@Test
+	@Deprecated
+	@SuppressWarnings("deprecation")
 	void testCustomizeRejectIllegalHeader() {
 		bind("server.tomcat.reject-illegal-header=false");
 		customizeAndRunServer((server) -> assertThat(