Use GraalVM for JDK 21 in GitHub workflow (google#2765)

GraalVM for JDK 17 does not receive any updates anymore
(at least not under the "GraalVM Free Terms and Conditions")

See https://github.com/graalvm/setup-graalvm/tree/v1.2.4?tab=readme-ov-file#notes-on-oracle-graalvm-for-jdk-17

Author: Marcono1234

.github/workflows/build.yml

@@ -34,7 +34,7 @@ jobs:
       - name: "Set up GraalVM"
         uses: graalvm/setup-graalvm@6f327093bb6a42fe5eac053d21b168c46aa46f22  # v1.2.4
         with:
-          java-version: '17'
+          java-version: '21'
           distribution: 'graalvm'
           # According to documentation in graalvm/setup-graalvm this is used to avoid rate-limiting issues
           github-token: ${{ secrets.GITHUB_TOKEN }}

test-graal-native-image/README.md

@@ -12,7 +12,7 @@ Technically it would also be possible to directly configure Native Image test ex
 
 ## Reflection metadata
 
-Native Image creation requires configuring which class members are accessed using reflection, see the [GraalVM documentation](https://www.graalvm.org/22.3/reference-manual/native-image/metadata/#specifying-reflection-metadata-in-json).
+Native Image creation requires configuring which class members are accessed using reflection, see the [GraalVM documentation](https://www.graalvm.org/jdk21/reference-manual/native-image/metadata/#specifying-reflection-metadata-in-json).
 
 The file [`reflect-config.json`](./src/test/resources/META-INF/native-image/reflect-config.json) contains this reflection metadata.