Deprecate the sparse_vector field type.

[jtibshirani]

We have not seen much adoption of this experimental field type, and don't see a
clear use case as it's currently designed. This PR deprecates the field type in
7.x. It will be removed from 8.0 in a follow-up PR.

We issue a deprecation warning both when parsing the sparse_vector mapping,
and when a sparse vector function is passed to a script.

[elasticmachine]

Pinging @elastic/es-search (:Search/Search)

[mayya-sharipova]

should we issue a deprecation logging only when parsing a sparse_vector mapping in public Mapper.Builder<?,?> parse(..) instead of issuing it every time when parsing a document with a sparse_vector?

[jtibshirani]

Yes, sorry I asked you to review a bit too early :) I still need to fix this, plus some failing docs tests.

[jtibshirani]

@mayya-sharipova this is now ready for another look, apologies again for the false start!

[jtibshirani]

@elasticmachine run elasticsearch-ci/packaging-sample-matrix

[mayya-sharipova]

@jtibshirani Thanks Julie, this looks good to me, as long as we have decided to remove sparse vectors in 8.0

[jtibshirani]

@elasticmachine run elasticsearch-ci/packaging-sample-matrix

[jtibshirani]

@elasticmachine run elasticsearch-ci/packaging-sample-matrix