Change getdown to work with bundled Java 17/21/25 JRE

Our heavily adapted version of getdown stems from a download of getdown 1.8.3 and, whilst being patched for (e.g. zipslip) vulnerabilities where needed, has not kept pace with the latest getdown (now maintained at https://github.com/bekoenig/getdown with significant upgrades to 2.0.1 version).
This latest version (2.0.1) compiles and passes tests under Java 21, but doesn't pass all tests under Java 25 (single zipfile-related failure, doesn't look difficult to fix).

Some comments on how to work with getdown and more recent LTS Java versions are here:
https://issues.jalview.org/browse/JAL-4646?focusedCommentId=28948&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-28948