Upgrade Proguard to a version that supports Java 18.

For unfortunate reasons, we have to add a separate (build-time) dependency on
proguard-core, code from which proguard-base relies on but neither includes
directly nor expresses as a dependency.

How is it in the year of our lord two thousand and twenty two, we are still
making such basic build errors in our Maven plugins and POM files?

launcher/pom.xml

@@ -85,7 +85,7 @@
       <plugin>
         <groupId>com.github.wvengen</groupId>
         <artifactId>proguard-maven-plugin</artifactId>
-        <version>2.3.1</version>
+        <version>2.5.1</version>
         <executions>
           <execution>
             <phase>package</phase>
@@ -96,12 +96,18 @@
           <dependency>
             <groupId>com.guardsquare</groupId>
             <artifactId>proguard-base</artifactId>
-            <version>7.0.0</version>
+            <version>7.2.1</version>
+            <scope>runtime</scope>
+          </dependency>
+          <dependency>
+            <groupId>com.guardsquare</groupId>
+            <artifactId>proguard-core</artifactId>
+            <version>9.0.0</version>
             <scope>runtime</scope>
           </dependency>
         </dependencies>
         <configuration>
-          <proguardVersion>7.0.0</proguardVersion>
+          <proguardVersion>7.2.1</proguardVersion>
           <outputDirectory>${project.build.directory}</outputDirectory>
           <outjar>${project.build.finalName}.jar</outjar>
           <injar>${project.build.finalName}.jar</injar>