set version 2025.4 as latest stable

[annastuchlik]

Required to release 2025.4.
Setting 2025.4 as the latest stable version will make it the default at https://docs.scylladb.com/manual/stable/.

Update https://github.com/scylladb/scylladb-docs-homepage/blob/main/docs/_static/data/manual_doc_versions.json