TypeMappingDescriptor<T> still supports setting AllField

[sladeos]

NEST/Elasticsearch.Net version:
7.0.0/7.0.0

Elasticsearch version:
7.2.0

Description of the problem including expected versus actual behavior:
TypeMappingDescriptor still has the AllField method even though using it results in a 400 when creating an index. The method should maybe have been marked Obsolete in the 6.X version and removed in 7.X.
Steps to reproduce:

1. Define a CreateIndexDescriptor and in the Map section set AllField to disabled
2. Attempt to create the index.

            var testDescriptor = new CreateIndexDescriptor("test")
                .Map(m => m
                    .AllField(af => af.Enabled(false))
                    .AutoMap());
            var result = this.client.Indices.Create(testDescriptor);

Provide DebugInformation (if relevant):
Root mapping definition has unsupported parameters: [_all : {enabled=false}]

[russcam]

Thanks for reporting @sladeos; this should have been removed for 7.x 😢

Will mark as [Obsolete()].

[russcam]

I've opened #3918 to address