Update min supported gradle version to 6.1

This PR also updates the min agp version we test with because older agp
versions are using gradle apis which doesn't exist in gradle 6.0+.

GH PR: #4155

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/targets/js/MultiplePluginDeclarationDetector.kt

@@ -43,11 +43,7 @@ private constructor() {
             val detector = MultiplePluginDeclarationDetector()
             instance = detector
 
-            if (isConfigurationCacheAvailable(gradle)) {
-                BuildFinishedListenerService.getInstance(gradle).onClose { instance = null }
-            } else {
-                gradle.buildFinished { instance = null }
-            }
+            BuildFinishedListenerService.getInstance(gradle).onClose { instance = null }
 
             return detector
         }

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/tasks/Tasks.kt

@@ -141,18 +141,10 @@ public class GradleCompileTaskProvider {
         sessionsDir = GradleCompilerRunner.sessionsDir(task.project)
         projectName = task.project.rootProject.name.normalizeForFlagFile()
         val modulesInfo = GradleCompilerRunner.buildModulesInfo(task.project.gradle)
-        buildModulesInfo = if (!isConfigurationCacheAvailable(task.project.gradle)) {
-            task.project.provider {
-                object : IncrementalModuleInfoProvider {
-                    override val info = modulesInfo
-                }
-            }
-        } else {
-            task.project.gradle.sharedServices.registerIfAbsent(
-                IncrementalModuleInfoBuildService.getServiceName(), IncrementalModuleInfoBuildService::class.java
-            ) {
-                it.parameters.info.set(modulesInfo)
-            }
+        buildModulesInfo = task.project.gradle.sharedServices.registerIfAbsent(
+            IncrementalModuleInfoBuildService.getServiceName(), IncrementalModuleInfoBuildService::class.java
+        ) {
+            it.parameters.info.set(modulesInfo)
         }
         path = task.path
         logger = task.logger

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/utils/compatibiltiy.kt

@@ -31,7 +31,7 @@ import org.gradle.api.tasks.bundling.AbstractArchiveTask
 import org.gradle.util.GradleVersion
 import java.io.File
 
-const val minSupportedGradleVersion = "5.3"
+const val minSupportedGradleVersion = "6.1"
 
 internal val Task.inputsCompatible: TaskInputs get() = inputs