Upgrade shadow plugin to 7.1.0

[k-mack]

Updates shadow to 7.1.0.

This upgrade may not be acceptable as it requires Gradle 7.0+.

A workaround for those that want to use shadow 7.1.0 with this plugin, but cannot upgrade to Gradle 7, can do:

buildscript {
    configurations.classpath {
        resolutionStrategy {
            dependencySubstitution {
                // Remove this when "io.vertx.vertx-plugin" upgrades its Shadow dependency
                substitute module("com.github.jengelman.gradle.plugins:shadow") using module("gradle.plugin.com.github.johnrengelman:shadow:7.1.0") because "the shadow bundled with io.vertx.vertx-plugin is older than I want"
            }
        }
    }
}

plugins {
    id 'io.vertx.vertx-plugin' version "1.3.0"
}