chore(prdsub): Upgrade NATS helm chart to v2.12.1

What

This MR upgrades NATS cluster on prdsub environment to v2.12.1 - the latest release. See release notes here: https://github.com/nats-io/k8s/releases/tag/nats-2.12.1.

Why

Aligning with upstream, accounting for a major release. Though sifting through the notes, the changes aren't big enough. Also tested on our pre-staging environment, see: !9131 (comment 2868774572) for more details.

Links to relevant issues

Related https://gitlab.com/gitlab-org/analytics-section/platform-insights/core/-/issues/96