ci(folio): run gradle on jdk 21 for the metro plugin

the metro gradle plugin folio builds with publishes org.gradle.jvm.version 21
and java 21 class files, so every leg failed at the folio build on a 17
runtime. local builds pass on jdk 25, which is why only ci saw it.

.github/workflows/folio.yml

@@ -49,11 +49,15 @@ jobs:
           go-version-file: go.mod
           cache: true
 
-      - name: Set up JDK 17
+      - name: Set up the JDKs
         uses: actions/setup-java@v4
         with:
           distribution: temurin
-          java-version: "17"
+          # The metro gradle plugin folio builds with needs a 21 runtime; the
+          # sidecar toolchain pins 17. Both are installed so gradle can pick.
+          java-version: |
+            17
+            21
 
       - name: Set up Android SDK
         uses: android-actions/setup-android@v3
@@ -134,11 +138,15 @@ jobs:
       - name: Install idb-companion, xcodegen and just
         run: brew install facebook/fb/idb-companion xcodegen just
 
-      - name: Set up JDK 17
+      - name: Set up the JDKs
         uses: actions/setup-java@v4
         with:
           distribution: temurin
-          java-version: "17"
+          # The metro gradle plugin folio builds with needs a 21 runtime; the
+          # sidecar toolchain pins 17. Both are installed so gradle can pick.
+          java-version: |
+            17
+            21
 
       # The iOS app builds its Kotlin framework through the folio gradle
       # project, which configures :app:androidApp and so needs an Android SDK
@@ -208,11 +216,15 @@ jobs:
           go-version-file: go.mod
           cache: true
 
-      - name: Set up JDK 17
+      - name: Set up the JDKs
         uses: actions/setup-java@v4
         with:
           distribution: temurin
-          java-version: "17"
+          # The metro gradle plugin folio builds with needs a 21 runtime; the
+          # sidecar toolchain pins 17. Both are installed so gradle can pick.
+          java-version: |
+            17
+            21
 
       - name: Set up bun
         uses: oven-sh/setup-bun@v2