simple: limit generic loop copy test to Go 1.20 and newer

In Go before 1.20, the compiler (but not go/types) rejects generic type
declarations inside functions. Our tests didn't fail, however, because
the version of Go 1.19 we were testing with had a bug that hid the
compiler error from go/packages, and we actually checked the file
successfully. Fixing that bug in Go (in
c045822e4d7491271c91359a4ff7f333ca5d1ea9) broke the test.

simple/testdata/src/example.com/CheckLoopCopy/copy_generics.go

@@ -1,4 +1,4 @@
-//go:build go1.18
+//go:build go1.20
 
 package pkg