Go back to com.github.johnrengelman.shadow and upgrade ASM to 9.6

[rpalcolea]

This should restore the old behavior and upgrading ASM to 9.6 adds support for JDK 21

Sample POM:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.netflix.spectator</groupId>
  <artifactId>spectator-reg-atlas</artifactId>
  <version>1.8.0-SNAPSHOT</version>
  <name>spectator-reg-atlas</name>
  <description>spectator-reg-atlas</description>
  <url>https://github.com/netflix/spectator</url>
  <licenses>
    <license>
      <name>The Apache Software License, Version 2.0</name>
      <url>http://www.apache.org/licenses/LICENSE-2.0.txt</url>
      <distribution>repo</distribution>
    </license>
  </licenses>
  <developers>
    <developer>
      <id>netflixgithub</id>
      <name>Netflix Open Source Development</name>
      <email>netflixoss@netflix.com</email>
    </developer>
  </developers>
  <scm>
    <url>ssh://git@github.com/netflix/spectator.git</url>
  </scm>
  <dependencies>
    <dependency>
      <groupId>com.netflix.spectator</groupId>
      <artifactId>spectator-api</artifactId>
      <version>1.8.0-SNAPSHOT</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>com.netflix.spectator</groupId>
      <artifactId>spectator-ext-ipc</artifactId>
      <version>1.8.0-SNAPSHOT</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-api</artifactId>
      <version>1.7.36</version>
      <scope>runtime</scope>
    </dependency>
    <dependency>
      <groupId>javax.inject</groupId>
      <artifactId>javax.inject</artifactId>
      <version>1</version>
      <scope>runtime</scope>
    </dependency>
  </dependencies>

</project>