Update WEBrick to v1.6.1

Ruby ships with WEBrick v1.6.0, but v1.6.1 contains a fix for a CVE:
https://bugs.ruby-lang.org/issues/17201

We only use WEBrick for Sidekiq exporter to serve internal metrics, so
this CVE shouldn't be a user-facing issue.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/296224