DeclarativeEnum for vulnerabilities/external_issue_link model

What does this MR do and why?

Contributes to: #340749

We have a DeclarativeEnum implementation, so using it for ee/app/models/vulnerabilities/external_issue_link.rb model.

Screenshots or screen recordings

How to set up and validate locally

bin/rspec ee/spec/requests/api/graphql/vulnerabilities/external_issue_links_spec.rb
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}

Test environment set up in 5.65651 seconds
...

Finished in 10.91 seconds (files took 12.13 seconds to load)
3 examples, 0 failures

And it should pass ✅

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