refactor: use Base64 instead of Base64Utils (#3858)

SpringBoot 3.4 will remove Base64Utils

Author: NaccOll

spring-boot-admin-server/src/main/java/de/codecentric/boot/admin/server/web/client/BasicAuthHttpHeaderProvider.java

@@ -17,12 +17,12 @@
 package de.codecentric.boot.admin.server.web.client;
 
 import java.nio.charset.StandardCharsets;
+import java.util.Base64;
 import java.util.Collections;
 import java.util.Map;
 
 import org.springframework.http.HttpHeaders;
 import org.springframework.lang.Nullable;
-import org.springframework.util.Base64Utils;
 import org.springframework.util.StringUtils;
 
 import de.codecentric.boot.admin.server.domain.entities.Instance;
@@ -86,7 +86,7 @@ public HttpHeaders getHeaders(Instance instance) {
 	}
 
 	protected String encode(String username, String password) {
-		String token = Base64Utils.encodeToString((username + ":" + password).getBytes(StandardCharsets.UTF_8));
+		String token = base64Encode((username + ":" + password).getBytes(StandardCharsets.UTF_8));
 		return "Basic " + token;
 	}
 
@@ -101,6 +101,14 @@ protected String encode(String username, String password) {
 		return null;
 	}
 
+	private static String base64Encode(byte[] src) {
+		if (src.length == 0) {
+			return "";
+		}
+		byte[] dest = Base64.getEncoder().encode(src);
+		return new String(dest, StandardCharsets.UTF_8);
+	}
+
 	@lombok.Data
 	@lombok.NoArgsConstructor
 	@lombok.AllArgsConstructor