Drop convention usage on accessing Java SourceSet

This is only valid for Gradle 7.1+, so new plugin variant was added -
"gradle71". For Gradle <=7.0 old convention approach is still used.

^KT-47047 In Progress

buildSrc/src/main/kotlin/GradleCommon.kt

@@ -44,7 +44,7 @@ enum class GradlePluginVariant(
 ) {
     GRADLE_MIN("main", "6.7", "6.9"),
     GRADLE_70("gradle70", "7.0", "7.0"),
-    //GRADLE_71("gradle71", "7.1", "7.1"),
+    GRADLE_71("gradle71", "7.1", "7.1"),
 }
 
 /**

buildSrc/src/main/kotlin/gradle-plugin-common-configuration.gradle.kts

@@ -55,10 +55,17 @@ tasks.named("jar") {
 }
 
 if (!kotlinBuildProperties.isInJpsBuildIdeaSync) {
-    // Used for Gradle 7.0+ versions
+    // Used for Gradle 7.0 version
     val gradle70SourceSet = createGradlePluginVariant(
         GradlePluginVariant.GRADLE_70,
         commonSourceSet = commonSourceSet
     )
     publishShadowedJar(gradle70SourceSet, commonSourceSet)
+
+    // Used for Gradle 7.1+ versions
+    val gradle71SourceSet = createGradlePluginVariant(
+        GradlePluginVariant.GRADLE_71,
+        commonSourceSet = commonSourceSet
+    )
+    publishShadowedJar(gradle71SourceSet, commonSourceSet)
 }

buildSrc/src/main/kotlin/gradle-plugin-dependency-configuration.gradle.kts

@@ -19,13 +19,20 @@ extensions.extraProperties["kotlin.stdlib.default.dependency"] = "false"
 val commonSourceSet = createGradleCommonSourceSet()
 reconfigureMainSourcesSetForGradlePlugin(commonSourceSet)
 
-// Used for Gradle 7.0+ versions
+// Used for Gradle 7.0 version
 createGradlePluginVariant(
     GradlePluginVariant.GRADLE_70,
     commonSourceSet = commonSourceSet,
     isGradlePlugin = false
 )
 
+// Used for Gradle 7.1+ versions
+createGradlePluginVariant(
+    GradlePluginVariant.GRADLE_71,
+    commonSourceSet = commonSourceSet,
+    isGradlePlugin = false
+)
+
 publishing {
     publications {
         register<MavenPublication>(DEFAULT_MAIN_PUBLICATION_NAME) {