[FIRRTL] Reject ref statements in 4.0.0+.

[dtzSiFive]

cc #6715 .

Continue to support for versions it's part of the FIRRTL spec.

It may make sense to simplify parser (and IR/passes a smidge) and not support at some point? The test mentioned in that issue (and corresponding parse-errors.fir) as well as the spec-conformance ref test (ext.fir) will can be adjusted if this is dropped entirely.

Cannot drop "internalPaths" and pass support until this is removed which also requires annotations that rely on it to be deprecated / removed.

[seldridge]

If not hard dropping this, moving the tests of the parsing of this into a feature-gated test would be good.