Don't use _type field for nested paths

[romseygeek]

The _type field is not only used to store types (obsolete in 8.0) but also
the nested paths of child documents. This overload prevents us from removing
the _type field and TypeFieldType.

This commit changes nested field mappers to use a _nested_path field instead,
with some backwards-compatibility shims to ensure that pre-8.0 indexes still use
_type.

Closes #24362

[elasticmachine]

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

[romseygeek]

@elasticmachine update branch

[jtibshirani]

@romseygeek I think it would be helpful to separate this PR into two:

• Switch to _nested_path instead of _type to store nested paths.
• Then follow-up with a change to remove TypeFieldMapper.

This helps in isolating these two big changes, which are related but still distinct. It's also not clear to me whether we can just remove TypeFieldMapper in 8.0 -- some of the conversations we've had around API versioning seem to suggest that we need logic to accept (but ignore) references to _type in the search request. Separating the PR into two would let us make progress on reviewing the _nested_path change while we get clarity on the API versioning questions.

[romseygeek]

I've opened #51100 to deal with just adding the nested field mapper.

[jtibshirani]

Thanks for splitting this up! I'll let someone else (maybe @jpountz?) review #51100, since I'm not as familiar with the history/ context there.