ci: Use clang-20 for sanitizer tasks #31793

A new clang version generally comes with bugfixes, new (sanitizer) features, and deprecations.

Upgrade the sanitizer tasks to use the new version.

This was also suggested in #31691 (comment)

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--006a51241073e994b41acfe9ec718e94-->

Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/31793.

<!--021abf342d371248e50ceaed478a90ca-->

Reviews

See the guideline for information on the review process.

If your review is incorrectly listed, please react with 👎 to this comment and the bot will ignore it on the next update.

<!--174a7506f384e20aa4161008e828411d-->

Conflicts

Reviewers, this pull request conflicts with the following ones:

• #31802 (Add bitcoin-{node,gui} to release binaries for IPC by Sjors)
• #30975 (ci: build multiprocess on most jobs by Sjors)
• #28710 (Remove the legacy wallet and BDB dependency by achow101)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

ACK fa5a02bcfa2f3769859332fddb8954d6217de7fc - tested 20 in some other infra, we just needed to fix the same deprecation warnings we'd seen, in cryptofuzz: https://github.com/fanquake/cryptofuzz/commit/09ca550c3ef7d6645869cb1ac014a4413d5388ba.