feat: upgrade build environment to JDK 21 and centralize CI configuration

- Create a composite GitHub Action `gradle-setup` to encapsulate code checkout, wrapper validation, JDK 21 setup, and Gradle caching logic.
- Update all GitHub workflows (`publish-core`, `codeql`, `scheduled-updates`, `release`, etc.) to utilize the new centralized `gradle-setup` action.
- Upgrade the project's primary JDK requirement from 17 to 21 across `jitpack.yml`, workflow files, and build-logic conventions.
- Refactor `KotlinAndroid.kt` and `build.gradle.kts` to target JVM 21 for the application while maintaining JVM 17 compatibility for published library modules (`api`, `model`, `proto`).
- Introduce a new `build-desktop` job in `reusable-check.yml` to verify desktop artifact assembly during CI.
- Implement dynamic `cache_read_only` detection in workflows to optimize Gradle cache usage across different branch types and merge groups.
- Update project documentation (`GEMINI.md`, `AGENTS.md`, `CONTRIBUTING.md`) to reflect the JDK 21 requirement and provide guidance on Robolectric configuration for the new version.

build-logic/convention/build.gradle.kts

@@ -25,14 +25,14 @@ plugins {
 
 group = "org.meshtastic.buildlogic"
 
-// Configure the build-logic plugins to target JDK 17
+// Configure the build-logic plugins to target JDK 21
 // This improves compatibility for developers building the project or consuming its libraries.
 java {
-    sourceCompatibility = JavaVersion.VERSION_17
-    targetCompatibility = JavaVersion.VERSION_17
+    sourceCompatibility = JavaVersion.VERSION_21
+    targetCompatibility = JavaVersion.VERSION_21
 }
 
-kotlin { compilerOptions { jvmTarget = JvmTarget.JVM_17 } }
+kotlin { compilerOptions { jvmTarget = JvmTarget.JVM_21 } }
 
 dependencies {
     // This allows the use of the 'libs' type-safe accessor in the Kotlin source of the plugins

build-logic/convention/src/main/kotlin/org/meshtastic/buildlogic/KotlinAndroid.kt

@@ -49,8 +49,13 @@ internal fun Project.configureKotlinAndroid(commonExtension: CommonExtension) {
             defaultConfig.targetSdk = targetSdkVersion
         }
 
-        compileOptions.sourceCompatibility = JavaVersion.VERSION_17
-        compileOptions.targetCompatibility = JavaVersion.VERSION_17
+        val javaVersion = if (project.name in listOf("api", "model", "proto")) {
+            JavaVersion.VERSION_17
+        } else {
+            JavaVersion.VERSION_21
+        }
+        compileOptions.sourceCompatibility = javaVersion
+        compileOptions.targetCompatibility = javaVersion
     }
 
     configureMokkery()
@@ -170,9 +175,10 @@ internal fun Project.configureKotlinJvm() {
 /** Configure base Kotlin options */
 private inline fun <reified T : KotlinBaseExtension> Project.configureKotlin() {
     extensions.configure<T> {
-        // Using Java 17 for better compatibility with consumers (e.g. plugins, older environments)
-        // while still supporting modern Kotlin features.
-        jvmToolchain(17)
+        val javaVersion = if (project.name in listOf("api", "model", "proto")) 17 else 21
+        // Using Java 17 for published modules for better compatibility with consumers (e.g. plugins, older environments),
+        // and Java 21 for the rest of the app.
+        jvmToolchain(javaVersion)
 
         if (this is KotlinMultiplatformExtension) {
             targets.configureEach {
@@ -201,7 +207,8 @@ private inline fun <reified T : KotlinBaseExtension> Project.configureKotlin() {
 
     tasks.withType<KotlinCompile>().configureEach {
         compilerOptions {
-            jvmTarget.set(JvmTarget.JVM_17)
+            val isPublishedModule = project.name in listOf("api", "model", "proto")
+            jvmTarget.set(if (isPublishedModule) JvmTarget.JVM_17 else JvmTarget.JVM_21)
             allWarningsAsErrors.set(warningsAsErrors)
             freeCompilerArgs.addAll(
                 // Enable experimental coroutines APIs, including Flow