Bump maven in /sources (#182)

Bumps maven from 3.9.3-eclipse-temurin-17-alpine to 3.9.4-eclipse-temurin-21-alpine.

---
updated-dependencies:
- dependency-name: maven
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

sources/Dockerfile

@@ -22,7 +22,7 @@
 #
 # Build stage.
 #
-FROM maven:3.9.3-eclipse-temurin-17-alpine AS build
+FROM maven:3.9.4-eclipse-temurin-21-alpine AS build
 WORKDIR /app
 
 # Copy local code to the container image.