Relax FIPS constraints on PyPi packages

What does this MR do and why?

Previously on a FIPS system if a PyPi package were uploaded that included md5_digest, the upload would fail with a 422 Unprocessible Entity error due to !87180 (merged). This commit relaxes the constraint and only fails if sha256_digest is not present.

Even on a FIPS system, running a Docker image such as python:latest doesn't ship with a FIPS-enabled OpenSSL. The change in https://github.com/pypa/twine/issues/776 doesn't omit md5_digest as a result.

Relates to #385477 (closed)

How to set up and validate locally

1. Use a FIPS-enabled kernel (https://docs.gitlab.com/ee/development/fips_compliance.html#setting-up-a-fips-enabled-development-environment).
2. Install a standard GitLab EE installation.
3. Clone https://gitlab.com/gitlab-com/support/toolbox/gitlab-smoke-tests/.
4. Run a pipeline in the repository.
5. Manually play pypi-repository job.

MR acceptance checklist

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