Bump node from 20.2-slim to 21.6-slim in /deploy/docker

Bumps node from 20.2-slim to 21.6-slim.

---
updated-dependencies:
- dependency-name: node
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

deploy/docker/Dockerfile

@@ -110,7 +110,7 @@ CMD [ "/bin/sh", "/lowcoder/node-service/entrypoint.sh" ]
 ##
 ## Build lowcoder client application
 ##
-FROM node:20.2-slim AS build-client
+FROM node:21.6-slim AS build-client
 COPY ./client /lowcoder-client
 WORKDIR /lowcoder-client
 RUN yarn --immutable