[Improvement] Introduce Maven Modernizer plugin to Pulsar

As discussed in dev mail list: https://lists.apache.org/thread.html/re99b342765b2cfb26f3da0eedfb78e7312de158bf6c7ed0085d32781%40<dev.pulsar.apache.org> We want to introduce Maven Modernizer plugin to Pulsar. This plugin helps detecting uses of legacy APIs which can be replaced with modern APIs that are often more performant, safer, and idiomatic than the legacy equivalents. We’ll break down the task by sub-module so anyone interested can work on a sub-module independently. We’ll start with checking violation with Java 8. Acceptance criteria will be passing existing CI. Help are welcome, please reply with module you want to work on so there won’t be conflict. And if change for single module is too big, please break down pr to reasonable size, say < 100 change so it’s easier to review. Please don’t close the master issue via comment.

<plugin>
    <groupId>org.gaul</groupId>
    <artifactId>modernizer-maven-plugin</artifactId>
    <configuration>
        <failOnViolations>true</failOnViolations>
        <javaVersion>8</javaVersion>
    </configuration>
    <executions>
        <execution>
            <id>modernizer</id>
            <goals>
                <goal>modernizer</goal>
            </goals>
        </execution>
    </executions>
</plugin>

• managed-ledger
• pulsar-common
• pulsar-broker-common
• pulsar-broker-auth-athenz
• pulsar-broker-auth-sasl
• pulsar-broker
• pulsar-client-api
• pulsar-client
• pulsar-client-admin-api
• pulsar-client-admin
• pulsar-client-tools
• pulsar-client-tools-test
• pulsar-client-auth-athenz
• pulsar-client-auth-sasl
• pulsar-websocket
• pulsar-proxy
• pulsar-discovery-service
• pulsar-zookeeper-utils
• pulsar-testclient
• pulsar-config-validation
• pulsar-transaction
• pulsar-functions
• pulsar-metadata
• pulsar-package-management

[tisonkun]

Concretely, we turn on the plugin for pulsar-broker, but ignore several patterns now for gradually handling them. I think @youzipi is actively working on this. We can help with reviewing.

Flaky tests hurt a lot 😦

[tisonkun]

@MarvinCai @codelipenghui @nodece should be closed by #17968.