build: update Jetty to 9.4.42.v20210604 and switch to Spring Boot's version

Jetty manual versioning was introduced in 98ad71f and now
I think we don't need it.

Author: php-coder

pom.xml

@@ -599,9 +599,6 @@
 
 		<javassist.version>3.21.0-GA</javassist.version>
 
-		<!-- Redefine default value from spring-boot-dependencies -->
-		<jetty.version>9.4.19.v20190610</jetty.version>
-		
 		<!-- Don't forget to update version in the ResourceUrl class -->
 		<jquery.version>1.9.1</jquery.version>