fix: support capitalized relabel actions to match with prometheus-operator behavior

[QuentinBisson]

PR Description

The prometheus operator ServiceMonitors, PodMonitors and Probes relabelings sections support capitalized actions like so: https://github.com/prometheus-operator/prometheus-operator/blob/ccfd2196833a350100658b9d645f0935e3b90a3d/example/prometheus-operator-crd/monitoring.coreos.com_podmonitors.yaml#L349

The issue is prometheus itself only supports lowercase actions so the prometheus-operator maintainers added https://github.com/prometheus-operator/prometheus-operator/blob/ccfd2196833a350100658b9d645f0935e3b90a3d/pkg/prometheus/resource_selector.go#L294 to lowercase actions.

This PR aims to reproduce this as incorrect values like "Replace" actually causes alloy to panic like so:

panic: relabel: unknown relabel action type "Replace"

goroutine 906 [running]:
github.com/prometheus/prometheus/model/relabel.relabel(0xc001f343f0, 0xc00a0239a0)
        /go/pkg/mod/github.com/grafana/[email protected]/model/relabel/relabel.go:314 +0x96e
github.com/prometheus/prometheus/model/relabel.ProcessBuilder(...)
        /go/pkg/mod/github.com/grafana/[email protected]/model/relabel/relabel.go:233
github.com/prometheus/prometheus/scrape.PopulateLabels(0xc00a0239a0, 0xc003018d88, 0x0)
        /go/pkg/mod/github.com/grafana/[email protected]/scrape/target.go:445 +0x510
github.com/prometheus/prometheus/scrape.TargetsFromGroup(0xc00a134000, 0xc003018d88, 0x0, {0x0?, 0xa474900?, 0x6b?}, 0xc00a0239a0)
        /go/pkg/mod/github.com/grafana/[email protected]/scrape/target.go:571 +0x1d5
github.com/prometheus/prometheus/scrape.(*scrapePool).Sync(0xc009f3e1a0, {0xc00a0d38c0, 0x21, 0xc002980f60?})
        /go/pkg/mod/github.com/grafana/[email protected]/scrape/scrape.go:391 +0x1e6
github.com/prometheus/prometheus/scrape.(*Manager).reload.func1(0xc00365ff60?, {0xc00a0d38c0?, 0x40a2105?, 0xc00300ca80?})
        /go/pkg/mod/github.com/grafana/[email protected]/scrape/manager.go:180 +0x25
created by github.com/prometheus/prometheus/scrape.(*Manager).reload in goroutine 300
        /go/pkg/mod/github.com/grafana/prometheus@v

Which issue(s) this PR fixes

Notes to the Reviewer

PR Checklist

• CHANGELOG.md updated
• Tests updated

[QuentinBisson]

@mattdurham do you think you could take a look at this one?

[dehaansa]

Change looks good, but there are extra entries in the changelog.

[QuentinBisson]

@dehaansa should be better now, I think I made a bad merge