Warn when using deprecated syntax

[clintongormley]

Currently, users have to read the release notes for every version to check what syntax/functionality has been deprecated. We should make it easier, by warning or even throwing an exception.

This can be controlled by the deprecations setting which accepts:

• warn - logs and returns a warning in the request body when deprecated syntax/functionality/end points are used
• error - throw an exception when deprecated syntax/functionality/end points are used
• ignore - silently ignore deprecations

The default setting should be warn, to inform users during development of problems with their code. The user can set it to error to be sure that they are catching all problems, and they can set it to ignore in production so that known usage of deprecated functionality does not obscure real problems.

[clintongormley]

Closed in favour of #11033

[clintongormley]

Reopening. We should always log deprecations (based on the deprecation logging settings).

For warn, we should add a "warnings" section to the response, which lists all warnings. Strict throws an exception.

The index.query.parse.strict setting (see #11996) should be repurposed and renamed to response.deprecations.level, and should accept ignore, warn, strict.

The parse fields behaviour should be changed to collect deprecations, instead of throwing an exception, so that all warnings can be collected.

[nik9000]

I know we send a header when some usersIs this done or just partly done?

[clintongormley]

I think this can be closed now - we have enough tooling around deprecations

[javanna]

The index.query.parse.strict setting (see #11996) should be repurposed and renamed to response.deprecations.level, and should accept ignore, warn, strict.

The parse fields behaviour should be changed to collect deprecations, instead of throwing an exception, so that all warnings can be collected.

Did we do any of these? I hope we did and I missed them :)

[nik9000]

The parse fields behaviour should be changed to collect deprecations, instead of throwing an exception, so that all warnings can be collected.

We did this one, but only in non-strict mode. It just collects deprecated stuff and sends Warning headers for them. I we're certainly talking about the other one in #19552.