Remove separate test configurations for JDK 21

^KT-58765

Now that JDK 21 was released as stable, we can remove separate test
configurations for testing the compiler with JDK 21 and reintegrate
the corresponding test data into standard test configurations.

compiler/tests-for-compiler-generator/tests/org/jetbrains/kotlin/test/generators/GenerateJUnit5CompilerTests.kt

@@ -26,10 +26,6 @@ fun generateJUnit5CompilerTests(args: Array<String>, mainClassName: String?) {
                 model("diagnostics/testsWithStdLib", excludedPattern = excludedCustomTestdataPattern)
             }
 
-            testClass<AbstractJdk21DiagnosticTest> {
-                model("diagnostics/testsWithJdk21", excludedPattern = excludedCustomTestdataPattern)
-            }
-
             testClass<AbstractDiagnosticUsingJavacTest> {
                 model("diagnostics/tests/javac", pattern = "^(.*)\\.kts?$", excludedPattern = excludedCustomTestdataPattern)
             }
@@ -218,14 +214,6 @@ fun generateJUnit5CompilerTests(args: Array<String>, mainClassName: String?) {
                 model("diagnostics/testsWithStdLib", excludedPattern = excludedCustomTestdataPattern)
             }
 
-            testClass<AbstractFirPsiJdk21DiagnosticTest> {
-                model("diagnostics/testsWithJdk21", excludedPattern = excludedCustomTestdataPattern)
-            }
-
-            testClass<AbstractFirLightTreeJdk21DiagnosticTest> {
-                model("diagnostics/testsWithJdk21", excludedPattern = excludedCustomTestdataPattern)
-            }
-
             testClass<AbstractFirPsiWithActualizerDiagnosticsTest>(suiteTestClassName = "FirOldFrontendMPPDiagnosticsWithPsiTestGenerated") {
                 model("diagnostics/tests/multiplatform", pattern = "^(.*)\\.kts?$", excludedPattern = excludedCustomTestdataPattern)
             }