ci: switch setup-java distribution from zulu to temurin

Zulu's CDN (cdn.azul.com) has been returning HTTP 520 for several
hours, breaking JDK 21 setup across multiple PRs (#3075, #3095) on
every Linux runner. actions/setup-java's internal retry budget isn't
enough to ride through it.

Temurin (Adoptium) publishes its JDKs as GitHub Releases assets, so
the download path uses GitHub's own CDN — completely independent
infrastructure. Java 21 is the same upstream OpenJDK build either way,
so Gradle, Compose Multiplatform, and jpackage behave identically.

Applied to all setup-java steps in build.yml, smoke-test-desktop.yml,
and create-release.yml.

.github/workflows/build.yml

@@ -24,7 +24,7 @@ jobs:
       - name: Set up JDK 21
         uses: actions/setup-java@v5
         with:
-          distribution: 'zulu'
+          distribution: 'temurin'
           java-version: 21
 
       # Remote Gradle build cache: writes on push to main, reads on PRs and
@@ -71,7 +71,7 @@ jobs:
       - name: Set up JDK 21
         uses: actions/setup-java@v5
         with:
-          distribution: 'zulu'
+          distribution: 'temurin'
           java-version: 21
 
       - name: Set up Gradle
@@ -203,7 +203,7 @@ jobs:
       - name: Set up JDK 21
         uses: actions/setup-java@v5
         with:
-          distribution: 'zulu'
+          distribution: 'temurin'
           java-version: 21
 
       - name: Set up Gradle
@@ -256,7 +256,7 @@ jobs:
       - name: Set up JDK 21
         uses: actions/setup-java@v5
         with:
-          distribution: 'zulu'
+          distribution: 'temurin'
           java-version: 21
 
       - name: Set up Gradle

.github/workflows/create-release.yml

@@ -55,7 +55,7 @@ jobs:
       - name: Set up JDK 21
         uses: actions/setup-java@v5
         with:
-          distribution: 'zulu'
+          distribution: 'temurin'
           java-version: 21
 
       - name: Resolve tag + version
@@ -223,7 +223,7 @@ jobs:
       - name: Set up JDK 21
         uses: actions/setup-java@v5
         with:
-          distribution: 'zulu'
+          distribution: 'temurin'
           java-version: 21
 
       - name: Resolve tag + version
@@ -346,7 +346,7 @@ jobs:
       - name: Set up JDK 21
         uses: actions/setup-java@v5
         with:
-          distribution: 'zulu'
+          distribution: 'temurin'
           java-version: 21
 
       - name: Cache gradle

.github/workflows/smoke-test-desktop.yml

@@ -30,7 +30,7 @@ jobs:
       - name: Set up JDK 21
         uses: actions/setup-java@v5
         with:
-          distribution: 'zulu'
+          distribution: 'temurin'
           java-version: 21
 
       - name: Set up Gradle
@@ -60,7 +60,7 @@ jobs:
       - name: Set up JDK 21
         uses: actions/setup-java@v5
         with:
-          distribution: 'zulu'
+          distribution: 'temurin'
           java-version: 21
 
       - name: Set up Gradle