Add `latest` column into `security_scans` table

What does this MR do?

This MR introduces a new column called latest for the security_findings table. Prior to this change, we were joining to the ci_builds table to get the latest records but due to sharding effort, this is not an option anymore.

This will fix #339810 (closed) and unblock !66735 (merged).

Database review

We could also introduce a compound index but the number of records to be filtered will be either zero or too low.

An example query to utilize this new column would be;

SELECT 
  * 
FROM 
  security_scans
WHERE 
  pipeline_id = $1 AND latest IS TRUE;

I will create another MR to populate the data for the existing records after this gets merged.

rake db:migrate:up VERSION=20210902171406

== 20210902171406 AddLatestColumnIntoTheSecurityScansTable: migrating =========
-- add_column(:security_scans, :latest, :boolean, {:default=>true, :null=>false})
   -> 0.0089s
== 20210902171406 AddLatestColumnIntoTheSecurityScansTable: migrated (0.0231s)

rake db:migrate:down VERSION=20210902171406

== 20210902171406 AddLatestColumnIntoTheSecurityScansTable: reverting =========
-- remove_column(:security_scans, :latest)
   -> 0.0049s
== 20210902171406 AddLatestColumnIntoTheSecurityScansTable: reverted (0.0716s)

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.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team