Set index in lowercase

[teoteo]

Description of the Change

The index_name is transformed in lowercase before the index creation in Elasticsearch.
Change suggested by @felipeelia #2166 (comment)

Alternate Designs

Benefits

Uppercase text in Elasticsearch aren’t allowed so this fix transforms the index name in lowercase before creating an index

Possible Drawbacks

Verification Process

1. Installed WordPress an uppercase folder so the baseurl was: https://elasticpress.test/UPPERCASE_FOLDER ;
2. installed ElasticPress plugin and activated;
3. configured ElasticPress to a self hosted Elasticsearch server;
4. syncronized -> failed to create index;
5. applied the fix;
6. syncronized -> index created;
7. created some contents: they are searchable and the index health page is OK

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

Closes: #2166

Changelog Entry

Fixed issue with uppercase in siteurl

[Rahmon]

LGTM 👍