Hi,
Firstly, when you said you put the jstl jars in your webapp, I hope you meant you put the jsf jars in there, because the jstl jars should go on the container's classpath.
Next, the WARNing you see from the ConstraintSecurityHandler means that you have defined security constraints in your web.xml that can leave some methods/paths exposed. Typically occurs when you have a security constraint that names only a few methods, or excludes only a few methods. The servlet spec requires us to warn you of this. See the servlet spec chapter 13.8 Security: Specifying Security Constraints for a good explanation and examples.
Finally, the error "java.util.ServiceConfigurationError: org.apache.juli.logging.Log: Provider org.eclipse.jetty.apache.jsp.JuliLog not a subtype" indicates that you have multiple conflicting jsp-related jars on the classpath. Check you webapp doesn't have some extra jsp jars in there.
The following jsp-related jars must be on the container classpath (I will use the names of the jars as they appear in the distro):
Everything in $jetty.home/lib/apache-jsp:
org.eclipse.jdt.ecj-3.12.3.jar
org.eclipse.jetty.apache-jsp-9.4.11.v20180605.jar
org.mortbay.jasper.apache-el-8.5.24.2.jar
org.mortbay.jasper.apache-jsp-8.5.24.2.jar
Everything in $jetty.home/lib/apache-jstl:
org.apache.taglibs.taglibs-standard-impl-1.2.5.jar
org.apache.taglibs.taglibs-standard-spec-1.2.5.jar
You will also need jetty-annotations.jar and the jetty-plus.jar on the container classpath.
If you're using jsf, you can put those jars into your WEB-INF/lib, or you can put them onto the container classpath - but if you do that, then you need to add a pattern for them to the org.eclipse.jetty.server.webapp.ContainerIncludeJarPattern so jetty knows to scan them for taglibs.