Use graphql search for project names for attach to new issue

In [group level] Attach vulnerabilities to new issue (#517664 - closed), the search within the project dropdown is done on the frontend within the projects already loaded. Since the graphql query supports searching on backend, switch this mechanism so the search term is sent to the backend to have better results. Mostly useful if you're got many projects in a group.

Implementation details

1. Use debounce for search term (use default debounce time constant)
2. Adapt graphql query
3. Update specs

Verification steps

1. Go to https://gitlab.com/groups/gitlab-org/govern/threat-insights-demos/verification-projects/-/security/vulnerabilities
2. Select some vulnerabilities
3. Select the action "Attach to new issue"
4. In the "Select project" dropdown, search for "Security reports"
5. Verify that an actual GraphQL call is made with that search term and the results match the term