/usr/share/pki/lib/servlet.jar is in pki-base-java 10.6.0-1ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/pki/lib/../../java/servlet-api-3.1.jar
/usr/share/pki/lib/../../java/servlet-api-3.1.jar