Index create request should support singular 'mapping'

[JoshMock]

The current indices create spec expects the term mapping rather than mappings. Adding as an alias to avoid breaking compatibility.

See elastic/elasticsearch-js#1753.

[github-actions]

Following you can find the validation results for the API you have changed.

API	Status	Request	Response
indices.create	🔴	673/679	679/679

You can validate this API yourself by using the make validate target.

[swallez]

If we go back in time, it was mapping for ages (even forever ?). So we should just rename the property, since no user was ever able to use this property, either for configuring or reading the settings.

Note also that aliases are used for parsing and not serializing. As it is, in strongly typed clients the property will still be serialized as mappings.

[JoshMock]

On closer inspection, the indices-create docs does use mappings at the root, and the doc linked in the original issue is referring to body.settings.mapping.

I don't know why there are two ways to provide a mapping in indices.create, but the current spec does seem to reflect this properly.