Nested mapping attribute and descriptor incorrectly accept "Store" property

[stevejgordon]

NEST/Elasticsearch.Net version: 7.10.1

Elasticsearch version: 7.10.0

Description of the problem including expected versus actual behavior:
When using attribute mappings to define a Nested type, the attribute accepts a Store property. Elasticsearch returns an error when attempting such a mapping. We should evaluate removing this property given that it is not supported and also check that it's removed from the fluent mapping also. This is

This relates to the discussion in #5170

Steps to reproduce:

1. Create a type which includes a Nested attribute for one of the properties.
2. Include Store = true in the attribute
3. Elasticsearch will return an error - "Mapping definition for [field] has unsupported parameters: [store : true]"

Expected behavior
NEST should not offer the store property for nested field mappings.

[russcam]

Related: #3145

[stevejgordon]

Thanks @russcam 👍

[stevejgordon]

Attribute mappings are no longer supported for v8.