Clearly state that invalid fields are ignored

[bgamari]

Previously we would issue a warning of the following form when
we encountered a field that isn't supported by the cabal-version
declared in the cabal file:

Warning: ghc-heap.cabal:30:3: The field "cmm-sources" is available
only since the Cabal specification version 3.0.

The message, however, doesn't tell the user the consequence of this
warning. Consequently a user could easily be confused when the build
subsequently fail.

Avoid this possibility by clearly stating that the field value will be
ignored.

---

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[bgamari]

This arose out of the discussion on #5819.

[phadej]

One need to --accept parser-tests

[phadej]

Merged in #6173