Prohibit using old JVM backend with LV >= 1.6

#KT-48928 Fixed

compiler/testData/cli/jvm/abiStabilityUnstableWithOldBackend.out

@@ -1,3 +1,4 @@
+warning: language version is automatically inferred to 1.5 when using the old JVM backend. Consider specifying -language-version explicitly, or remove -Xuse-old-backend
 warning: -Xuse-old-backend is deprecated and will be removed in a future release
 error: -Xabi-stability=unstable is not supported in the old JVM backend
 COMPILATION_ERROR

compiler/testData/cli/jvm/bothJvmIrAndOldBackend.args

@@ -3,3 +3,5 @@ $TESTDATA_DIR$/simple.kt
 $TEMP_DIR$
 -Xuse-ir
 -Xuse-old-backend
+-language-version
+1.5

compiler/testData/cli/jvm/classAndTraitClash.args

@@ -2,3 +2,5 @@ $TESTDATA_DIR$/classAndTraitClash.kt
 -d
 $TEMP_DIR$
 -Xuse-old-backend
+-language-version
+1.5

compiler/testData/cli/jvm/oldBackendLv16.args

@@ -0,0 +1,6 @@
+$TESTDATA_DIR$/simple.kt
+-d
+$TEMP_DIR$
+-Xuse-old-backend
+-language-version
+1.6

compiler/testData/cli/jvm/oldBackendLv16.out

@@ -0,0 +1,2 @@
+error: old JVM backend does not support language version 1.6 or above. Please use language version 1.5 or below, or remove -Xuse-old-backend
+COMPILATION_ERROR

compiler/testData/cli/jvm/suspendInFunInterfaceIrDisabled.args

@@ -2,3 +2,5 @@ $TESTDATA_DIR$/suspendInFunInterfaceIrDisabled.kt
 -d
 $TEMP_DIR$
 -Xuse-old-backend
+-language-version
+1.5