Pi-hole v5 shipped with lighttpd as a dependency, to be installed upon user confirmation.
It is not unlikely that you only run it because you've installed Pi-hole, but I can't tell whether other software you've installed took advantage of it as well.
Pi-hole v6 comes with its own embedded webserver. As far as Pi-hole is concerned, removing lighttpd won't affect it, and often is the correct thing to do.
But only you can know whether your other software would still depend on it.
If you decide it isn't, see lighttpd still running after upgrade.
Once it's gone, you could then consider moving Pi-hole's webserver back to port 80, e.g. via webserver.port under All settings ยป Miscellaneous, or via CLI:
sudo pihole-FTL --config webserver.port "80o,8443os,[::]:80o,[::]:8443os"
Above would still keep port 8443 for HTTPS, as in your case HTTPS's standard 443 is claimed by nginx.