Build: Setup jvmTarget 1.6 for several common modules

Some modules depending on them target jvm 1.6 and can't depend on
jvm 1.8 module in Gradle 6.5.

libraries/kotlin.test/annotations-common/build.gradle

@@ -4,6 +4,8 @@ apply plugin: 'kotlin-platform-common'
 
 configurePublishing(project)
 
+jvmTarget = "1.6"
+
 dependencies {
     compile kotlinStdlib("common")
     testCompile project(":kotlin-test:kotlin-test-common")

libraries/kotlin.test/common/build.gradle

@@ -4,6 +4,8 @@ apply plugin: 'kotlin-platform-common'
 
 configurePublishing(project)
 
+jvmTarget = "1.6"
+
 dependencies {
     compile kotlinStdlib("common")
     testCompile project(":kotlin-test:kotlin-test-annotations-common")

libraries/stdlib/common/build.gradle

@@ -4,6 +4,8 @@ apply plugin: 'kotlin-platform-common'
 
 configurePublishing(project)
 
+jvmTarget = "1.6"
+
 def commonSrcDir = "../src"
 def commonTestSrcDir = "../test"