Search API process index not using native Elasticsearch filters

Problem/Motivation

Using certain Search API index processors are not working on proper way. Certain processors are e.g. Stemmer, Stopwords.

Steps to reproduce

1. Create a node with title talking
2. Create new Search API (Elasticsearch) index using content as datasource.
3. Add title field on index fields. /admin/config/search/search-api/index/INDEX_NAME/fields
4. Add Tokenizer and Stemmer processors. Make sure Tokenizer is before Stemmer. /admin/config/search/search-api/index/INDEX_NAME/processors
5. Clear the index and Index the content on /admin/config/search/search-api/index/INDEX_NAME
6. On Elasticsearch the title field is stored as talk not talking.

Proposed resolution

Many possible resolutions:
1. Use native Elasticsearch stemmer filters/analyzers. https://www.drupal.org/project/search_api_elasticsearch_synonym → and https://www.drupal.org/project/elasticsearch_connector_autocomp → use EventSubscriberInterface to properly utilize analyzers.
2. Document that Search API Stemmer and Stopwords do not work as expected when using Elasticsearch.
3. Disable Stemmer checkbox on Elasticsearch based indexes on /admin/config/search/search-api/index/INDEX_NAME/processors.

Remaining tasks

1. Decide best approach.
2. Implement solution.

User interface changes

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API changes

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Data model changes

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