security.txt: Add acknowledgments page to honour our security researches
The page will be linked in [https://torproject.org/.well-known/security.txt]
Details: https://tools.ietf.org/html/draft-foudil-securitytxt-04#section-3.4.1 Basically a place to honour the work of former / current security researchers.
This could also go into #25475 (moved).
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