ci: drop JDK 21 from backend build matrix

JPDFium's published artifacts carry org.gradle.jvm.version=25 (FFM is
finalized at JDK 22+; JPDFium picks 25 as the source target). Gradle's
variant attribute matching refuses to resolve the dep on JDK 21
targets, so the 21 cells in this matrix were guaranteed to fail.

Now that targetCompatibility is bumped to 25 across the project, JDK 25
is the only supported toolchain.

.github/workflows/backend-build.yml

@@ -18,7 +18,10 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        jdk-version: [21, 25]
+        # JDK 25 is the floor: JPDFium's published artifacts require
+        # org.gradle.jvm.version=25 (FFM is final at 22+, JPDFium picked 25
+        # as its source target). Earlier JDKs can't even resolve the dep.
+        jdk-version: [25]
         spring-security: [true, false]
     steps:
       - name: Harden Runner