pom.xml: fix Maven warning about RELEASE version of TestNG

Maven didn't like the use of the `RELEASE` metaversion, so we're
replacing it with the concrete latest version of TestNG.

For reference, here's the warning in the output of `mvn package` on
Apache Maven 3.9.5:

```
[WARNING]
[WARNING] Some problems were encountered while building the effective model for io.kaitai:kaitai-struct-runtime:jar:0.11-SNAPSHOT
[WARNING] 'dependencies.dependency.version' for org.testng:testng:jar is either LATEST or RELEASE (both of them are being deprecated) @ line 151, column 16
[WARNING]
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING]
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
[WARNING]
```

Also see
https://cwiki.apache.org/confluence/display/MAVEN/Maven+3.x+Compatibility+Notes#Maven3.xCompatibilityNotes-PluginMetaversionResolution

> ### Plugin Metaversion Resolution
>
> Internally, Maven 2.x used the special version markers `RELEASE` and
> `LATEST` to support automatic plugin version resolution. These
> metaversions were also recognized in the `<version>` element for a
> `<plugin>` declaration. For the sake of reproducible builds, Maven 3.x
> no longer supports usage of these metaversions in the POM. As a
> result, users will need to replace occurrences of these metaversions
> with a concrete version.

Author: generalmimon

pom.xml

@@ -145,10 +145,11 @@
   </distributionManagement>
 
   <dependencies>
+    <!-- https://mvnrepository.com/artifact/org.testng/testng -->
     <dependency>
       <groupId>org.testng</groupId>
       <artifactId>testng</artifactId>
-      <version>RELEASE</version>
+      <version>7.8.0</version>
       <scope>test</scope>
     </dependency>
   </dependencies>