slack-username not respected

`slack-username` is not respected any more.

```cli
❯ git grep -i "slack.*username"
README.md: --slack-username string slack username for reboot notfications (default "kured")
charts/kured/README.md:| `configuration.slackUsername` | cli-parameter `--slack-username` | `""` |
charts/kured/templates/daemonset.yaml: {{- if .Values.configuration.slackUsername }}
charts/kured/templates/daemonset.yaml: - --slack-username={{ .Values.configuration.slackUsername }}
charts/kured/values.minikube.yaml: # slackUsername: "" # slack username for reboot notfications (default "kured")
charts/kured/values.yaml: slackUsername: "" # slack username for reboot notfications (default "kured")
cmd/kured/main.go: slackUsername string
cmd/kured/main.go: rootCmd.PersistentFlags().StringVar(&slackUsername, "slack-username", "kured",
cmd/kured/main.go: "slack username for notifications")
kured-ds.yaml:# - --slack-username=prod

kured on  main via 🐹 v1.16.12
❯
```

This came up after #368 (along with #469).

I'm a bit unsure what to do. I feel that going with `shoutrrr` is the right thing to do, but some "old" setups won't work they used to any more.

Start with the --slack-username flag in cmd/kured/main.go and trace how reboot notifications consume it. Review the README.md and Helm chart references, then compare the current behavior with the older setups mentioned in #368 and #469. Done requires a decided compatibility behavior and verification that the configured username is respected.

Written by the indexing model from the issue text.