feat!: Java 21 for build and runtime (#7177)

* build: Update jacoco version to 0.8.11
* build: Enforce Java 21 and above check for build
* CI: Use Java 21 in Github CI workflows
* CI: Use Java 21 in circleci workflows
* build: Update gradle verification metadata for jacoco 0.8.11
* refactor: Fix javadoc related warnings which are applicable to Java 21
* fix(test): BackwardSyncAlgSpec slightly increase timeout to pass it in CI

---------

Signed-off-by: Usman Saleem <usman@usmans.info>

util/src/main/java/org/hyperledger/besu/util/EndianUtils.java

@@ -18,6 +18,7 @@ package org.hyperledger.besu.util;
 public class EndianUtils {
   // next two methods adopted from:
   // https://github.com/bcgit/bc-java/blob/master/core/src/main/java/org/bouncycastle/util/Pack.java
+  private EndianUtils() {}
 
   /**
    * Long to big endian.

util/src/main/java/org/hyperledger/besu/util/FutureUtils.java

@@ -25,6 +25,8 @@ import java.util.function.Supplier;
 /** The Future utils. */
 public class FutureUtils {
 
+  private FutureUtils() {}
+
   /**
    * Returns a new CompletionStage that, when the provided stage completes exceptionally, is
    * executed with the provided stage's exception as the argument to the supplied function.

util/src/main/java/org/hyperledger/besu/util/log/LogUtil.java

@@ -28,6 +28,8 @@ public class LogUtil {
   static final String BESU_NAMESPACE = "org.hyperledger.besu";
   static final int MAX_SUMMARY_DEPTH = 20;
 
+  private LogUtil() {}
+
   /**
    * Throttles logging to a given logger.
    *

util/src/main/java/org/hyperledger/besu/util/platform/PlatformDetector.java

@@ -43,6 +43,8 @@ public class PlatformDetector {
   private static String _glibc;
   private static String _jemalloc;
 
+  private PlatformDetector() {}
+
   /**
    * Gets OS type.
    *