Not compatible with uppercase collection identifiers

[StijnCaerts]

Describe the bug
It is not possible to add Items for collections with uppercase characters in the collection identifier.

To Reproduce
Steps to reproduce the behavior:

1. Add a collection with uppercase characters in the collection identifier.
2. Add items to this collection
3. Error response:

HTTP 500 Internal Server Error
{"code":"RequestError","description":"RequestError(400, 'invalid_index_name_exception', 'Invalid index name [items_UPPERCASE_collection_id], must be lowercase')"

Expected behavior
While the STAC best practices mention the use of lower case characters for Item IDs, it should be possible to have collections with uppercase characters in their collection identifier.