Increase PBKDF2 iterations to OWASP recommended value

[Leon338]

This PR increases the PBKDF2 iterations to align with the OWASP recommended value for better security. The change is minimal and does not affect other parts of the codebase.

• Updated PBKDF2 iterations to 600'000
• Verified tests pass successfully
• Discussion reference: crypto: 100,000 iterations of PBKDF2 is insecure #1346

This change follows security best practices as recommended by OWASP. Let me know if any further modifications are needed.

[github-actions]

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[Leon338]

I have read the CLA Document and I hereby sign the CLA

[Leon338]

I ran some benchmarks on Cloudflare Workers, and based on the cpuTimeMs observed in production, it should be possible to run PBKDF2 with 600,000 iterations in under 250ms on production. On average, this takes about 6× longer than running 100,000 iterations. Given these results, this implementation should not pose an issue for the live system.

Of course, this doesn’t work on Workers Free, but 100,000 iterations didn’t work there either.