As described in https://github.com/NixOS/nixpkgs/pull/394017,
grafana-agent does not build with Go 1.23 anymore, and Go 1.22 has been
removed.
grafana-agent has been deprecated by Grafana (in favour of Grafana
Alloy), and will be EOL on 2025-11-01, which would be part of the
upcoming 25.05 release.
Instead of leaving us with a broken package, drop it alltogether, and
add release notes.
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