Disable finding link creation

What does this MR do?

This MR disables finding link creation by putting it behind a feature flag. This is necessary so that we can fix the data creation issue and clear the existing data without adding more invalid data to the table. This is part of the implementation plan to fix the bug reported in #330882 (closed).

Screenshots or Screencasts (strongly suggested)

N/A

How to setup and validate locally (strongly suggested)

1. Disable the vulnerability_finding_replace_metadata flag

   Feature.enable(:vulnerability_finding_replace_metadata)

2. Check the number of Vulnerabilities::FindingLink's in the database

   Vulnerabilities::FindingLink.count

3. On the master branch, run a pipeline with a dependency scan (suggested repo: https://gitlab.com/gitlab-examples/security/security-reports)

4. Check the number of Vulnerabilities::FindingLink's in the database. It should be the same as before

   Vulnerabilities::FindingLink.count

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #330882 (closed)