chore(lint): enable gosec (#7792)

Enable "gosec" linter.

Exclude:

- All G115 (integer overflow) findings, to be fixed separately.

Add targeted gosec annotations for:

- non-crypto math/rand usage
- md5 used only for file change detection
- G114 ("net/http serve with no timeout settings"), to be fixed
  separately.

Other findings fixed.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

plugin/health/health.go

@@ -63,6 +63,7 @@ func (h *health) OnStartup() error {
 	ctx := context.Background()
 	ctx, h.stop = context.WithCancel(ctx)
 
+	// #nosec G114 -- TODO
 	go func() { http.Serve(h.ln, h.mux) }()
 	go func() { h.overloaded(ctx) }()