Just bump the JVM toolchain version to 21

I don't want to install a JDK 17 just for this project. Use and merge the previous commit if there is really a problem.

Author: ShreckYe

buildSrc/src/main/kotlin/common-conventions.gradle.kts

@@ -21,7 +21,7 @@ kotlin {
     // for `androidx.compose`
 
     jvm() // TODO: `jvm("desktop")`?
-    jvmToolchain(17)
+    jvmToolchain(21)
 
     //androidTarget()