refactor(logging): replace the logrus backend with log/slog (#1304)

Log through `log/slog`, with a console `slog.Handler` that writes the text the runner has always written (sorted `key=value` fields on a plain stream, a colored prefix on a terminal), so nothing changes for users. I compared its output byte for byte with logrus's `TextFormatter` over plain, colored, timestamped and caller cases.

Why not `slog.TextHandler`: its output differs from what users and their log parsers see today, and `ReplaceAttr` cannot close the gap.
- level is `WARN` where logrus wrote `warning`, and time has milliseconds and no quotes
- fields keep call order instead of being sorted
- the caller is one `source=` key, where the runner writes `func=` and `file=` (reshaped by `callPrettyfier`), and `ReplaceAttr` returns a single attribute
- no colored prefix on a terminal
- no hook to rewrite a whole formatted line, which the process-wide secret masking relies on

Libraries that log through logrus's global logger (the workflow model, the Docker client) are forwarded to the process-wide logger, so they keep following its level, format and secret masking. This is why logrus stays in `go.mod`.

Stacked on https://gitea.com/gitea/runner/pulls/1303.

Assisted-by: Codet
Co-authored-by: bircni <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1304
Reviewed-by: bircni <[email protected]>

internal/app/cmd/daemon_test.go

@@ -188,3 +188,10 @@ func TestShouldRetryDeclare(t *testing.T) {
 		require.Equal(t, tc.retry, shouldRetryDeclare(tc.err), tc.err.Error())
 	}
 }
+
+// A dependency that reconfigures logrus's global logger when it is loaded makes its own log lines
+// show up twice or not at all, so this fails when upgrading one starts doing that. It does not
+// cover a dependency that does so later, while running.
+func TestLogrusBridgeSurvivesDependencies(t *testing.T) {
+	require.True(t, logging.LogrusBridgeIntact())
+}