build: update jitpack JDK used for build to 21

When I bumped the e2e and unit test github runners to 21 I did not
also bump the jitpack build, but I believe it is best to have them
all using the same build toolchain

.github/workflows/tests_emulator.yml

@@ -50,7 +50,7 @@ jobs:
         uses: actions/setup-java@v4
         with:
           distribution: "temurin"
-          java-version: "21"
+          java-version: "21"  # also change jitpack.yml
 
       - name: Setup Gradle
         uses: gradle/actions/setup-gradle@v3

.github/workflows/tests_unit.yml

@@ -56,7 +56,7 @@ jobs:
         uses: actions/setup-java@v4
         with:
           distribution: "temurin"
-          java-version: "21"
+          java-version: "21"  # also change jitpack.yml
 
       - name: Setup Gradle
         uses: gradle/actions/setup-gradle@v3

jitpack.yml

@@ -1,9 +1,14 @@
 # We want control over the SDK used to build since we need newer JDKs
+# Use the latest LTS supported by temurin with version format xx.yy.zz-tem (ignore sub-version)
+# You may find versions e.g. for JDK21 like so https://adoptium.net/temurin/archive/?version=21
+# You may verify that a JDK is installed by looking at the "sdk list java" output from, for example:
+# https://jitpack.io/com/github/ankidroid/Anki-Android/v2.18alpha7/build.log (just use a recent tag,
+# if it has not built yet you will have to wait for it to build then on refresh the build log should exist)
 before_install:
     - sdk update
     - sdk list java
-    - sdk install java 17.0.9-tem
-    - sdk use java 17.0.9-tem
+    - sdk install java 21.0.2-tem
+    - sdk use java 21.0.2-tem
 
 # We can do the absolute minimum to build the API module, no need to build AnkiDroid module
 install: