[kotlin.reflect] Introduce ClassValue-based cache for KClassImpl

* Replace pcollections with ClassValue/ConcurrentHashMap-based caches
* Do not store weak references, instead cache strong references and count on ClassValue to unload the corresponding classloader if necessary
* ConcurrentHashMap does not rely on WeakReference as it's only selected on Android where classloader leaks don't exist
* Update reflect/scripting JDK requirement to Java 8 in order to proceed

#KT-53454
#KT-50705


Merge-request: KT-MR-6788
Merged-by: Vsevolod Tolstopyatov <qwwdfsad@gmail.com>

libraries/scripting/common/build.gradle.kts

@@ -3,7 +3,7 @@ plugins {
     id("jps-compatible")
 }
 
-project.updateJvmTarget("1.6")
+project.updateJvmTarget("1.8")
 
 dependencies {
     api(kotlinStdlib())

libraries/scripting/dependencies-maven/build.gradle.kts

@@ -3,7 +3,7 @@ plugins {
     id("jps-compatible")
 }
 
-project.updateJvmTarget("1.6")
+project.updateJvmTarget("1.8")
 
 dependencies {
     implementation(kotlinStdlib())

libraries/scripting/dependencies/build.gradle.kts

@@ -3,7 +3,7 @@ plugins {
     id("jps-compatible")
 }
 
-project.updateJvmTarget("1.6")
+project.updateJvmTarget("1.8")
 
 dependencies {
     api(kotlinStdlib())

libraries/scripting/jvm/build.gradle.kts

@@ -3,7 +3,7 @@ plugins {
     id("jps-compatible")
 }
 
-project.configureJvmToolchain(JdkMajorVersion.JDK_1_6)
+project.configureJvmToolchain(JdkMajorVersion.JDK_1_8)
 
 dependencies {
     api(project(":kotlin-script-runtime"))