ElasticSearch user mapping: first name and last name should use a normalizer to allow case-insensitive sort

[naman9271]

Fixes: #1199
This pull request updates the Elasticsearch configuration in openml_OS/libraries/ElasticSearch.php to improve the indexing and search capabilities for user names. The main changes are the addition of new fields for first and last names with case-insensitive keyword indexing, and the introduction of a custom normalizer to ensure consistent lowercase storage and querying.

Elasticsearch mapping improvements:

• Added first_name and last_name fields to the mapping, both using the keyword type and a lowercase_normalizer for case-insensitive searches.

Elasticsearch analysis configuration:

• Introduced a lowercase_normalizer in the index analysis settings, defined as a custom normalizer with a lowercase filter, to support the new case-insensitive keyword fields.