PYTHON-5439 Remove dead link in PyMongo 4 migration guide (#2428) #7772

Job	Run time
Link Check	2m 48s
Make an sdist	15s
Typing Tests (3.11)	1m 2s
CPython 3.13t-ubuntu-latest	9m 10s
Typing Tests (3.9)	1m 5s
CPython 3.9-ubuntu-latest	9m 3s
CPython pypy-3.10-ubuntu-latest	10m 18s
CPython 3.13-ubuntu-latest	8m 53s
Test using minimum dependencies and supported Python	30s
DocTest	49s
Docs Checks	47s
Test async's minimum dependencies and Python	40s
static	3m 5s
Install from SDist and Test	40s
	49m 5s

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