GET /search filter-lang doesn't default to cql2-text

[StijnCaerts]

Describe the bug
The GET /search filter-lang parameter doesn't default to cql2-text, like it is defined in the specification of the filter extension. Instead, cql2-json is used as the default when no value is supplied.
https://github.com/stac-api-extensions/filter?tab=readme-ov-file#get-query-parameters-and-post-json-fields

https://github.com/stac-utils/stac-fastapi-elasticsearch/blob/main/stac_fastapi/elasticsearch/stac_fastapi/elasticsearch/core.py#L383-L388

To Reproduce
Steps to reproduce the behavior:

1. Perform a GET /search request with the filter parameter (eg. filter=cloud_cover < 10).
2. The filter cannot be parsed and an error is returned.

Expected behavior
The default filter-lang in a GET search is cql2-text and queries can be parsed successfully.