… linting errors (#11305)

**What this PR does / why we need it**:

This PR removes the `if .Values.ruler.enabled` condition from the
`loki-egress-alertmanager` networkpolicy as this was causing linting
issues (i.e nil pointer) whenever enabling the networkpolicies since the
`ruler` field is not present in the values.

From what I know, the ruler is now part of the backend, thus I also
replaced the `rulerSelectorLabels` by the `backendSelectorLabels`.

**Special notes for your reviewer**:

**Checklist**
- [x] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [ ] Documentation added
- [ ] Tests updated
- [x] `CHANGELOG.md` updated
- [ ] If the change is worth mentioning in the release notes, add
`add-to-release-notes` label
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/setup/upgrade/_index.md`
- [x] For Helm chart changes bump the Helm chart version in
`production/helm/loki/Chart.yaml` and update
`production/helm/loki/CHANGELOG.md` and
`production/helm/loki/README.md`. [Example
PR](https://github.com/grafana/loki/commit/d10549e3ece02120974929894ee333d07755d213)
- [ ] If the change is deprecating or removing a configuration option,
update the `deprecated-config.yaml` and `deleted-config.yaml` files
respectively in the `tools/deprecated-config-checker` directory.
[Example
PR](https://github.com/grafana/loki/pull/10840/commits/0d4416a4b03739583349934b96f272fb4f685d15)

---------

Signed-off-by: QuantumEnigmaa <thibaud@giantswarm.io>
Co-authored-by: Michel Hollands <42814411+MichelHollands@users.noreply.github.com>