fix: update OWASP Testing Guide link to v4.2 (#6928)

Fixes OWASP Testing Guide 4.0 link is invalid #6921
Address lint: Broken links reported from urlchecker #5470

books/free-programming-books-subjects.md

@@ -725,7 +725,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Intrusion Detection Systems with Snort](http://ptgmedia.pearsoncmg.com/images/0131407333/downloads/0131407333.pdf) (PDF)
 * [OpenSSL Cookbook](https://www.feistyduck.com/library/openssl-cookbook/)
 * [OWASP Mobile Security Testing Guide](https://mobile-security.gitbook.io/mobile-security-testing-guide/) - Bernhard Mueller et al.
-* [OWASP Testing Guide 4.0](https://www.owasp.org/images/1/19/OTGv4.pdf) (PDF)
+* [OWASP Testing Guide 4.2](https://owasp.org/www-project-web-security-testing-guide/v42/) - The OWASP® Foundation (HTML, [PDF](https://github.com/OWASP/wstg/releases/download/v4.2/wstg-v4.2.pdf))
 * [OWASP Top 10 for .NET Developers](http://www.troyhunt.com/2011/12/free-ebook-owasp-top-10-for-net.html)
 * [Security Engineering](http://www.cl.cam.ac.uk/~rja14/book.html)
 * [The Joy of Cryptography (2021)](https://web.engr.oregonstate.edu/~rosulekm/crypto/) - Mike Rosulek (PDF) (:construction: *in process*)