Remove OpsGenie collector

[TheoBrigitte]

Since we are no longer using OpsGenie this PR does remove the OpsGenie collector, along with its configuration, dependencies, and related documentation.

It's all gone now:

$ grep -ri opsgenie
CHANGELOG.md:- Remove OpsGenie collector

[TheoBrigitte]

This seems to run, even though there are some Github rate limit errors.

$ k get po
NAME                                        READY   STATUS    RESTARTS   AGE
operational-load-exporter-864949f8f-5zkfk   2/2     Running   0          2m44s
$ k logs -f deploy/operational-load-exporter|hl -P
Defaulted container "operational-load-exporter" out of: operational-load-exporter, grafana-agent
2026/01/05 10:44:41 GET https://api.github.com/search/issues?q=is:issue+repo:giantswarm%2Fgiantswarm+is:open+label:postmortem+label:team%2Frainbow&page=1&per_page=1: 403 API rate limit of 30 still exceeded until 2026-01-05 10:45:32 +0000 UTC, not making remote request. [rate reset in 50s]
2026/01/05 10:44:41 GET https://api.github.com/search/issues?q=is:issue+repo:giantswarm%2Fgiantswarm+is:open+label:postmortem+label:team%2Fup&page=1&per_page=1: 403 API rate limit of 30 still exceeded until 2026-01-05 10:45:32 +0000 UTC, not making remote request. [rate reset in 50s]