Bump library/node in /backstage

Bumps library/node from 20-bookworm-slim to 25-bookworm-slim.

---
updated-dependencies:
- dependency-name: library/node
  dependency-version: 25-bookworm-slim
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

backstage/Dockerfile

@@ -1,4 +1,4 @@
-FROM docker.io/library/node:20-bookworm-slim AS build
+FROM docker.io/library/node:25-bookworm-slim AS build
 
 WORKDIR /app
 
@@ -44,7 +44,7 @@ COPY container_utils/default.conf.template /etc/nginx/templates/default.conf.tem
 
 EXPOSE 80
 
-FROM docker.io/library/node:20-bookworm-slim as backend
+FROM docker.io/library/node:25-bookworm-slim as backend
 
 WORKDIR /app