Gradle build update

[PhilipRoman]

Description

• Bump javac to 1.7 to match pom.xml
• Use new plugin block syntax
• Replace deprecated 'maven' plugin with 'maven-publish' (as described here: https://docs.gradle.org/6.8.3/userguide/upgrading_version_5.html#legacy_publication_system_is_deprecated_and_replaced_with_the_publish_plugins)
• replace deprecated 'compile' and 'testCompile' configurations with 'implementation' and 'testImplementation', respectively. (Note: this assumes that slf4j is an internal dependency and it's API is not exposed to users of this library)

Motivation and Context

Ensures that this project can be built using Gradle

How Has This Been Tested?

Built on jdk-8 and jdk-15, with Gradle 6.8.3. I manually verified that the maven-publish plugin works and produces the following POM file:

<?xml version="1.0" encoding="UTF-8"?>
<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <modelVersion>4.0.0</modelVersion>
  <groupId>org.java-websocket</groupId>
  <artifactId>Java-WebSocket</artifactId>
  <version>1.5.2-SNAPSHOT</version>
  <dependencies>
    <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-api</artifactId>
      <version>1.7.25</version>
      <scope>compile</scope>
    </dependency>
  </dependencies>
</project>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.