chore: update emulator version to 1.5.31 (#119)

Co-authored-by: GitHub Actions <actions@github.com>

Author: RoryQ

Dockerfile

@@ -4,7 +4,7 @@ COPY go.mod go.sum main.go ./
 ENV CGO_ENABLED=0
 RUN go build .
 
-FROM gcr.io/cloud-spanner-emulator/emulator:1.5.30 AS runtime
+FROM gcr.io/cloud-spanner-emulator/emulator:1.5.31 AS runtime
 COPY backend/query backend/query
 COPY --from=builder /build/spanner-emulator ./
 EXPOSE 9010 9020