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

gradlew.bat

@@ -13,8 +13,10 @@
 @rem See the License for the specific language governing permissions and
 @rem limitations under the License.
 @rem
+@rem SPDX-License-Identifier: Apache-2.0
+@rem
 
-@if "%DEBUG%" == "" @echo off
+@if "%DEBUG%"=="" @echo off
 @rem ##########################################################################
 @rem
 @rem  Gradle startup script for Windows
@@ -25,7 +27,8 @@
 if "%OS%"=="Windows_NT" setlocal
 
 set DIRNAME=%~dp0
-if "%DIRNAME%" == "" set DIRNAME=.
+if "%DIRNAME%"=="" set DIRNAME=.
+@rem This is normally unused
 set APP_BASE_NAME=%~n0
 set APP_HOME=%DIRNAME%
 
@@ -40,13 +43,13 @@ if defined JAVA_HOME goto findJavaFromJavaHome
 
 set JAVA_EXE=java.exe
 %JAVA_EXE% -version >NUL 2>&1
-if "%ERRORLEVEL%" == "0" goto execute
+if %ERRORLEVEL% equ 0 goto execute
 
-echo.
-echo ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH.
-echo.
-echo Please set the JAVA_HOME variable in your environment to match the
-echo location of your Java installation.
+echo. 1>&2
+echo ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH. 1>&2
+echo. 1>&2
+echo Please set the JAVA_HOME variable in your environment to match the 1>&2
+echo location of your Java installation. 1>&2
 
 goto fail
 
@@ -56,11 +59,11 @@ set JAVA_EXE=%JAVA_HOME%/bin/java.exe
 
 if exist "%JAVA_EXE%" goto execute
 
-echo.
-echo ERROR: JAVA_HOME is set to an invalid directory: %JAVA_HOME%
-echo.
-echo Please set the JAVA_HOME variable in your environment to match the
-echo location of your Java installation.
+echo. 1>&2
+echo ERROR: JAVA_HOME is set to an invalid directory: %JAVA_HOME% 1>&2
+echo. 1>&2
+echo Please set the JAVA_HOME variable in your environment to match the 1>&2
+echo location of your Java installation. 1>&2
 
 goto fail
 
@@ -75,13 +78,15 @@ set CLASSPATH=%APP_HOME%\gradle\wrapper\gradle-wrapper.jar
 
 :end
 @rem End local scope for the variables with windows NT shell
-if "%ERRORLEVEL%"=="0" goto mainEnd
+if %ERRORLEVEL% equ 0 goto mainEnd
 
 :fail
 rem Set variable GRADLE_EXIT_CONSOLE if you need the _script_ return code instead of
 rem the _cmd.exe /c_ return code!
-if  not "" == "%GRADLE_EXIT_CONSOLE%" exit 1
-exit /b 1
+set EXIT_CODE=%ERRORLEVEL%
+if %EXIT_CODE% equ 0 set EXIT_CODE=1
+if not ""=="%GRADLE_EXIT_CONSOLE%" exit %EXIT_CODE%
+exit /b %EXIT_CODE%
 
 :mainEnd
 if "%OS%"=="Windows_NT" endlocal