Set supported validator version on DXC/disable_paq.hlsl (#6493)

This test is failing in the internal validator back-compat suite because
the feature is not supported on older validators.

tools/clang/test/DXC/disable_paq.hlsl

@@ -1,3 +1,4 @@
+// REQUIRES: dxil-1-8
 // RUN: %dxc -T lib_6_7 -disable-payload-qualifiers %s 2>&1 | FileCheck %s
 // RUN: %dxc -T lib_6_8 -disable-payload-qualifiers %s 2>&1 | FileCheck %s