fix: replace Java toolchain with sourceCompatibility for fdroid compat

fdroid build environment has only JDK 21, toolchain auto-provisioning
is disabled. Use explicit sourceCompatibility/targetCompatibility instead.

gradle.properties

@@ -14,5 +14,5 @@ org.gradle.jvmargs=-Xmx2048m -Dfile.encoding=UTF-8
 # Kotlin code style for this project: "official" or "obsolete":
 kotlin.code.style=official
 # Default app version for local builds. Release workflow overrides this from the git tag.
-appVersionName=2.6.7
+appVersionName=2.6.8
 android.useAndroidX=true

xray-protos/build.gradle.kts

@@ -4,9 +4,8 @@ plugins {
 }
 
 java {
-    toolchain {
-        languageVersion.set(JavaLanguageVersion.of(11))
-    }
+    sourceCompatibility = JavaVersion.VERSION_11
+    targetCompatibility = JavaVersion.VERSION_11
 }
 
 protobuf {