docs(release-notes): backfill 9.16 entries from master commits

Add 14 release-note entries that were merged on master but not yet
captured in the 9.16 notes: 1 new feature (#2431), 6 housekeeping
(#9817, #9866, #9917, #9959, #10014, #10023) and 7 bug fixes (#9701,
#9782, #9933, #9952, #9985, #10013, #10030). Entries inserted in
numeric order within each section.

Also add an "Additional changes (no associated issue) -> Dependencies"
section mirroring the 9.15 format, listing net direct dep bumps
between REL-9_15 and HEAD across Python (requirements.txt,
tools/requirements.txt, web/regression/requirements.txt), web/
package.json, and runtime/package.json. Transitive yarn resolutions
and the setuptools pin (covered by bug fix #9829) are excluded.