chore(no-release): refactor to slog.DiscardHandler

Oleksandr Redkorequested to merge
gitlab-community/gitlab-org/api/client-go:chore/discard-handler into main

Refactor tests and comments to use slog.DiscardHandler and enable the sloglint linter, which detects such cases.

The slog.DiscardHandler is available starting from Go 1.24.

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