Fix dead links in README

[1t1sCooL]

Link-checked the README and fixed what is confirmed dead. Every dead URL was re-verified with curl (browser UA; NXDOMAIN double-checked via 8.8.8.8), and every replacement URL returns HTTP 200.

• https://leanpub.com/nodejssecurity → https://leanpub.com/essential-nodejs-security
  • Old slug 404; same book by Liran Tal now at leanpub.com/essential-nodejs-security (HTTP 200, title 'Essential Node.js Security [Leanpub PDF/iPad/Kindle]').
• https://newsletter.nodejs-security.com/ → https://www.nodejs-security.com/newsletter
  • Subdomain NXDOMAIN (dig @8.8.8.8 empty); same newsletter now at www.nodejs-security.com/newsletter (HTTP 200, title 'Node.js Security Newsletter' on Liran Tal's site).
• https://ossindex.sonatype.org/rest → https://guide.sonatype.com/
  • /rest returns 404; ossindex.sonatype.org itself redirects to guide.sonatype.com which states 'OSS Index ... now lives inside Sonatype Guide' (HTTP 200) - official successor of the OSSIndex service the link text refers to.
• https://www.datadoghq.com/product/application-security-monitoring/ → https://www.datadoghq.com/product/app-and-api-protection/
  • Old product URL 404; the ex-Sqreen ASM product was renamed to 'App and API Protection', live at /product/app-and-api-protection/ (HTTP 200, title 'App and API Protection | Datadog').
• https://www.zdnet.com/google-amp/article/malicious-npm-packages-caught-installing-remote-access-trojans/ → https://www.zdnet.com/article/malicious-npm-packages-caught-installing-remote-access-trojans/
  • AMP path returns HTTP 410; canonical non-AMP article URL on the same site returns 200 with exact title 'Malicious npm packages caught installing remote access trojans | ZDNET'.

Happy to adjust or split this if you'd prefer a different fix for any item.