Update nats_exporter from 0.16.0 to 0.20.0

[prometheus-rpm-bot]

https://github.com/nats-io/prometheus-nats-exporter/releases/tag/v0.20.0
Release notes:

## What's Changed
* build: add a makefile target to .gitignore by @philpennock in https://github.com/nats-io/prometheus-nats-exporter/pull/409
* Bump actions/setup-go from 6.3.0 to 6.4.0 by @dependabot[bot] in https://github.com/nats-io/prometheus-nats-exporter/pull/411
* Bump docker/bake-action from 6.10.0 to 7.0.0 by @dependabot[bot] in https://github.com/nats-io/prometheus-nats-exporter/pull/406
* Bump docker/setup-qemu-action from 3.7.0 to 4.0.0 by @dependabot[bot] in https://github.com/nats-io/prometheus-nats-exporter/pull/405
* Bump docker/setup-buildx-action from 3.12.0 to 4.0.0 by @dependabot[bot] in https://github.com/nats-io/prometheus-nats-exporter/pull/403
* Bump Go version by @Jarema in https://github.com/nats-io/prometheus-nats-exporter/pull/412
* Bump docker/login-action from 3.7.0 to 4.1.0 by @dependabot[bot] in https://github.com/nats-io/prometheus-nats-exporter/pull/413
* Bump docker/bake-action from 7.0.0 to 7.1.0 by @dependabot[bot] in https://github.com/nats-io/prometheus-nats-exporter/pull/414
* Add last_delivery and last_ack metrics by @tierpod in https://github.com/nats-io/prometheus-nats-exporter/pull/410
* Bump dependencies by @piotrpio in https://github.com/nats-io/prometheus-nats-exporter/pull/415

## New Contributors
* @tierpod made their first contribution in https://github.com/nats-io/prometheus-nats-exporter/pull/410

**Full Changelog**: https://github.com/nats-io/prometheus-nats-exporter/compare/v0.19.2...v0.20.0