Split out Jira issue creation to a separate section

What does this MR do and why?

This change is behind a feature flag jira_multiple_project_keys. In a previous MR !146087 (merged), we added Jira project keys (multiple) field which is to be decoupled from Jira project key (single) field.

This MR splits the current Issues section into 2 sections:

• View Jira issues (optional): This now contains Enable Jira issues checkbox and Jira project keys input (Note: multiple project keys).
• Jira issue creation from vulnerabilities (optional): This now contains Jira project key input (Note: single project key) and all fields around Jira issue creation from vulnerabilities.

There is a follow-up to polish some of the UI aspects in #452161 (closed).

MR acceptance checklist

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Screenshots or screen recordings

Before (without feature flag)	Before (with feature flag)	After (with feature flag)

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:jira_multiple_project_keys)

2. Go to Project > Settings > Integrations > Jira. For example, http://127.0.0.1:3000/gitlab-org/gitlab-test/-/settings/integrations/jira/edit.

Related to #440430 (closed)