Exclude ED25519 keys from QA FIPS tests

What does this MR do and why?

FIPS systems don't support ED25519 keys, so skip the use of these deploy keys in QA.

Relates to #367429 (closed)

How to set up and validate locally

On a FIPS system:

FIPS=1 QA_GITLAB_URL=https://fips.example.com GITLAB_USERNAME=root GITLAB_PASSWORD=REDACTED bundle exec rspec qa/specs/features/browser_ui/6_release/deploy_key/clone_using_deploy_key_spec.rb

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