Update stackprof to v0.2.21

What does this MR do and why?

There are only minor changes in this release, but for FIPS systems MD5 has been replaced with SHA256 (https://github.com/tmm1/stackprof/pull/177).

Diff: https://my.diffend.io/gems/stackprof/0.2.15/0.2.21

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.