doc: replace outdated OpenSSL comment in test README
doc: replace outdated OpenSSL comment in test README

Summary: This is a backport of Core [[https://github.com/bitcoin/bitcoin/pull/16960 | PR16960]]

Test Plan: Proof-reading

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8072

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