Increase tenant migration donor service thread pool max size

The tenant migration donor service has a thread pool with a max size of 8 threads (configurable by a server parameter) that is shared by all instances running a tenant migration. Until SERVER-54208 is finished, when an instances copies cluster time keys from the recipient, it will briefly block a thread in that pool. The number of keys to copy should be small (by default one document is generated every 90 days), but to guarantee the donor service can support 50 concurrent migrations like the recipient service (as per SERVER-53168) and to reduce the time pressure for SERVER-54208, the max number of threads in the donor service pool should be increased until SERVER-54208 is completed.