[build] Set Kotlin compiler back to 1.8.10

(Mistake by dca39f9b607a323b266d3485be52a861928a0bf1)

buildSrc/src/main/kotlin/Versions.kt

@@ -31,7 +31,7 @@ object Versions {
     val consoleIntellij get() = "223-$project-172-1" // idea-mirai-kotlin-patch
     val consoleTerminal get() = project
 
-    const val kotlinCompiler = "1.8.20-RC"
+    const val kotlinCompiler = "1.8.10"
     const val kotlinStdlib = kotlinCompiler
     const val dokka = "1.8.10"