Disable Gravatar if FIPS enabled

What does this MR do and why?

Describe in detail what your merge request does and why.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/361354

We need to disable Gravatar for FIPS mode #19495 (comment 899933825).

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How to set up and validate locally

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• Specs can also be checked locally with the FIPS_MODE env variable like so:

FIPS_MODE=true bundle exec rspec spec/helpers/avatars_helper_spec.rb

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