Bump to Gradle 8.10.2, require Java 11

Bump Gradle from 6.8.3 to 8.10.2 and increase the minimum required
Java version from 8 to 11 (SMACK-953).

The switch from Java 8 to 11 caused some Bytecode portability issues
regarding NIO Buffers. Java changed with version 9 the return type of
some subclasses of Buffer to return the specific Buffer type instead
of the Buffer superclass [JDK-4774077]. For example, ByteBuffer.filp()
previously returned Buffer, while it does return ByteBuffer now.

This sensible change was not reflected by the Android API [1], which
means that AnimalSniffer rightfully started to complain that there is
no method "ByteBuffer ByteBuffer.flip()" in Android, there is only
"Buffer ByteBuffer.flip()", and those are incompatible methods on
Java's Bytecode layer.

As workaround, this changes

    return charBuffer.flip().toString();

to

    ((java.nio.Buffer) charBuffer).flip();
    return charBuffer.toString();

to restore the Bytecode portability between Android and Java.

Errorprone also got new checks, of which JavaUtilDate and JdkObsolete
are wroth mentioning.

JavaUtilData basically strongly recommends to use Java's newer time
API over java.util.Date. But since Smack was Java 8 until now,
j.u.Date is widely used.

Similar JdkObsolete mentions obsolete JDK APIs, like data structures
like Vector and Stack. But mostly LinkedList, which should usually be
replaced by ArrayList. And this is what this commit largely does.

JDK-4774077: https://bugs.openjdk.org/browse/JDK-4774077
1: https://issuetracker.google.com/issues/369219141

smack-debug/build.gradle

@@ -1,3 +1,7 @@
+plugins {
+	id 'org.igniterealtime.smack.java-common-conventions'
+}
+
 description = """\
 Smack GUI debugger.
 Inspect the exchanged XMPP stanzas."""

smack-debug/src/main/java/org/jivesoftware/smackx/debugger/EnhancedDebugger.java

@@ -160,6 +160,7 @@ public class EnhancedDebugger extends SmackDebugger {
     private ReaderListener readerListener;
     private WriterListener writerListener;
 
+    @SuppressWarnings("JavaUtilDate")
     private Date creationTime = new Date();
 
     // Statistics variables
@@ -756,6 +757,7 @@ public class EnhancedDebugger extends SmackDebugger {
      * @param dateFormatter the SimpleDateFormat to use to format Dates
      * @param packet        the read stanza to add to the table
      */
+    @SuppressWarnings("JavaUtilDate")
     private void addReadPacketToTable(final SimpleDateFormat dateFormatter, final TopLevelStreamElement packet) {
         SwingUtilities.invokeLater(new Runnable() {
             @Override
@@ -827,6 +829,7 @@ public class EnhancedDebugger extends SmackDebugger {
      * @param dateFormatter the SimpleDateFormat to use to format Dates
      * @param packet        the sent stanza to add to the table
      */
+    @SuppressWarnings("JavaUtilDate")
     private void addSentPacketToTable(final SimpleDateFormat dateFormatter, final TopLevelStreamElement packet) {
         SwingUtilities.invokeLater(new Runnable() {
             @Override

smack-debug/src/main/java/org/jivesoftware/smackx/debugger/EnhancedDebuggerWindow.java

@@ -204,7 +204,7 @@ public final class EnhancedDebuggerWindow {
      * Creates the main debug window that provides information about Smack and also shows
      * a tab panel for each connection that is being debugged.
      */
-    @SuppressWarnings({ "rawtypes", "unchecked" })
+    @SuppressWarnings({ "rawtypes", "unchecked", "JdkObsolete" })
     private void createDebug() {
 
         frame = new JFrame("Smack Debug Window");