Drop REQUIRE(size <= 65536); from lib/isc/netmgr/tcp.c:tcp_alloc_cb()

https://gitlab.isc.org/isc-private/bind9/-/merge_requests/181#note_146512:

"A suggested size (65536 at the moment in most cases) is provided, but it’s just an indication, not related in any way to the pending data to be read. The user is free to allocate the amount of memory they decide." We can set buf->len to 64k always, and get rid of this require (ditto for udp).