chore: bump x/sync to 0.7

[siretart]

Note that this version changes the behavior of semaphore slightly. After https://cs.opensource.google/go/x/sync/+/14be23e5b48bec28285f8a694875175ecacfddb3 the semaphore package will reliably raise errors for both go routines

change the test to test for a substring to be more relaxed in case that behavior gets reverted