Remove the legacy query planner (PR #6418)
The legacy query planner has been removed in this release. In the previous release, Router 1.58, it was already no longer used by default but it was still available through the experimental_query_planner_mode configuration key. That key is now removed.
Also removed are configuration keys which were only relevant to the legacy planner:
supergraph.query_planning.experimental_parallelism: the new planner can always use available parallelism.supergraph.experimental_reuse_query_fragments: this experimental algorithm that attempted to reuse fragments from the original operation while forming subgraph requests is no longer present. Instead, by default new fragment definitions are generated based on the shape of the subgraph operation.
By @SimonSapin in #6418