Fix stderr to also be captured by log4j (#51569)

In #50259 we redirected stdout and stderr to log4j, to capture jdk
and external library messages. However, a typo in the method name used
to redirect the stream in java means stdout is currently being
duplicated twice, and stderr not captured. This commit corrects that
mistake. Unfortunately this is at a level that cannot really be tested,
thus we are still missing tests for this behavior.

server/src/main/java/org/elasticsearch/common/logging/LogConfigurator.java

@@ -249,7 +249,7 @@ public FileVisitResult visitFile(final Path file, final BasicFileAttributes attr
         // third party libraries may do that
         System.setOut(new PrintStream(new LoggingOutputStream(LogManager.getLogger("stdout"), Level.INFO),
             false, StandardCharsets.UTF_8.name()));
-        System.setOut(new PrintStream(new LoggingOutputStream(LogManager.getLogger("stderr"), Level.WARN),
+        System.setErr(new PrintStream(new LoggingOutputStream(LogManager.getLogger("stderr"), Level.WARN),
             false, StandardCharsets.UTF_8.name()));
     }