loki: Simplify loki.yaml

Loki v2.4.0 introduced a common config section[1][2] making the config
much smaller and better defaults.

So stripped loki.yaml down, removed options where the default is "good
enough" and tweaked retention settings per the upgrade notes[3].

So the config should basically be the example loki.yaml[4] + compactor,
retention enabled and a few tweaks (ex: compress chunks with zstd).

[1] https://github.com/grafana/loki/releases/tag/v2.4.0
[2] https://grafana.com/docs/loki/latest/configuration/#common_config
[3] https://grafana.com/docs/loki/latest/upgrading/#the-single-binary-no-longer-runs-a-table-manager
[4] https://github.com/grafana/loki/blob/v2.4.0/cmd/loki/loki-local-config.yaml