Migrating AppEngine application to Java 11 runtime - [http://java.sun.com/jsp/jstl/core] cannot be resolved

Question

So I have an AppEngine application that I'm working on migrating to the Java 11 runtime, and am using the appengine-simple-jetty-main artifact, as outlined in https://cloud.google.com/appengine/docs/standard/java-gen2/war-packaging, to replace the built-in Jetty server of the old Java 8 runtime.

Now if I use mvn jetty:run to bring up my application locally, everything works fine. But when I try to run the appengine-simple-jetty-main wrapper via mvn exec:java -Dexec.args="<my application directory>/target/<application>.war", I hit an issue.

The server starts up fine, and executes various ServletContextListeners that use outside dependencies declared in my pom.xml. I'm also able to navigate to static HTML. When I try to navigate to a JSP Servlet, however, I'm getting the error

HTTP ERROR 500 org.apache.jasper.JasperException: The absolute uri: [http://java.sun.com/jsp/jstl/core] cannot be resolved in either web.xml or the jar files deployed with this application

And likewise, I get this same error when I actually deploy to AppEngine.

As mentioned before, this works just fine when I bypass appengine-simple-jetty-main wrapper using mvn jetty:run, and my pom.xml includes the dependency

<dependency>
    <groupId>jakarta.servlet.jsp.jstl</groupId>
    <artifactId>jakarta.servlet.jsp.jstl-api</artifactId>
    <version>2.0.0</version>
</dependency>

My build section looks like

    <build>
        <plugins>
            <plugin>
                <groupId>org.eclipse.jetty</groupId>
                <artifactId>jetty-maven-plugin</artifactId>
                <version>11.0.11</version>
            </plugin>
            <plugin>
                <groupId>com.google.cloud.tools</groupId>
                <artifactId>appengine-maven-plugin</artifactId>
                <version>2.4.3</version>
                <configuration>
                    <deploy.projectId>contra-program</deploy.projectId>
                    <deploy.version>GCLOUD_CONFIG</deploy.version>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-dependency-plugin</artifactId>
                <version>3.1.1</version>
                <executions>
                    <execution>
                        <id>copy</id>
                        <phase>prepare-package</phase>
                        <goals>
                            <goal>copy-dependencies</goal>
                        </goals>
                        <configuration>
                            <outputDirectory>${project.build.directory}/appengine-staging</outputDirectory>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>

So I'm a bit flummoxed. Other maven dependencies seem to be getting brought in just fine. It's only the JSTL library that's having issues (so far, at least), and even then, only when I use the appengine-simple-jetty-main wrapper.

The nearest existing question I could find was cannot load JSTL taglib within embedded Jetty server, but I haven't had luck with the provided solutions (though I'm more than happy to admit the possibility that I'm just not applying one or more of them correctly).

Any pointers would be greatly appreciated :-)