dashboard/app: purge at most 10 crashes at once
dashboard/app: purge at most 10 crashes at once

We see some episodic
"failed to delete old crash texts: Call error 11: Deadline exceeded (timeout)"
errors in logs.
Deleting at most 10 bugs should be enough since we do this check
after adding each new crash.

zoulasc/syzkaller,xairy/syzkaller,dotweiba/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,dotweiba/syzkaller,xairy/syzkaller,google/syzkaller,zoulasc/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,dotweiba/syzkaller,zoulasc/syzkaller,zoulasc/syzkaller,dotweiba/syzkaller,google/syzkaller,google/syzkaller