Cosmos: fully remove sync I/O

[roji]

The Cosmos SDK does not provide sync APIs, only async; the Cosmos EF provider was originally implemented with sync-over-async, and we obsoleted and disabled that behavior by default in EF 9 (see https://learn.microsoft.com/en-us/ef/core/what-is-new/ef-core-9.0/breaking-changes#cosmos-nosync).

For EF 11, we should remove sync-over-async altogether, as users will have had a full LTS release (10) with the obsoletion.