Retroactively add CHANGES entry 5624 for BIND 9.16.17
While trying to wrap my mind around all the v9.16 backports related
to network manager and cleaning up CHANGES, I failed to account for
the fact that CHANGES entry 5638 originally had different contents in
main and v9_16:
Down the line, this ultimately resulted in #2638 (closed) getting excluded from
CHANGES in v9_16, which (with some "help" from a leftover MR, !4924 (closed))
caused some confusion as to when v9_16 started running taskmgr tasks
in netmgr loops.
#2638 (closed) is a big change, so I suggest to retroactively add its
CHANGES entry to the relevant section of that file in v9_16. I plan
to do that next week, when I merge July release branches back into
maintenance branches.