Use Elasticsearch 8 as default for `ElasticsearchDocumentStore`

[bogdankostic]

With #5027, we added support for Elasticsearch 8 in ElasticsearchDocumentStore. As a transition phase, ElasticsearchDocumentStore will still use Elasticsearch 7 as default for Haystack 1.19 and 1.20. Starting from Haystack 1.21, we want Elasticsearch 8 to be the default for `ElasticsearchDocumentStore. For this, we should make the following changes:

• Change elasticsearch extra to install Elasticsearch 8 dependencies
• Adapt failed import message for elasticsearch/es7.py.
• Adapt required extra in elasticsearch/es8.py
• Adapt Elasticsearch version in launch_es