Issue 75: Keycloak client 23.0.4 upgrade changes

[divyank-lakhera]

This PR updates Keycloak version to 23.0.4

To make successful Gradle build,

• Updated Gradle to 8.6
• Updated com.github.johnrengelman.shadow to 8.0.0

sles15sp4:~/pravega-keycloak/pravega-keycloak # ./gradlew build
Downloading https://services.gradle.org/distributions/gradle-8.6-bin.zip
..............................................................................................................................

Welcome to Gradle 8.6!

Here are the highlights of this release:
 - Configurable encryption key for configuration cache
 - Build init improvements
 - Build authoring improvements

For more details see https://docs.gradle.org/8.6/release-notes.html

Starting a Gradle Daemon, 1 stopped Daemon could not be reused, use --status for details

Deprecated Gradle features were used in this build, making it incompatible with Gradle 9.0.

You can use '--warning-mode all' to show the individual deprecation warnings and determine if they come from your own scripts or plugins.

For more on this, please refer to https://docs.gradle.org/8.6/userguide/command_line_interface.html#sec:command_line_warnings in the Gradle documentation.

BUILD SUCCESSFUL in 1m 38s
8 actionable tasks: 2 executed, 6 up-to-date

Testing: Flink longevity apps are running smoothly.

[crazyzhou]

LGTM