[Devops Adoption] Fix security scan query timeouts

What does this MR do?

Tries to fix query timeout which occurred for security scan query.

Migration output

== 20210708124229 AddSecurityScansCreatedAtIndex: reverting ===================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:security_scans, :created_at, {:algorithm=>:concurrently})
   -> 0.0102s
-- remove_index(:security_scans, {:algorithm=>:concurrently, :column=>:created_at})
   -> 0.0102s
== 20210708124229 AddSecurityScansCreatedAtIndex: reverted (0.0225s) ==========

== 20210708124229 AddSecurityScansCreatedAtIndex: migrating ===================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:security_scans, :created_at, {:algorithm=>:concurrently})
   -> 0.0082s
-- add_index(:security_scans, :created_at, {:algorithm=>:concurrently})
   -> 0.0110s
== 20210708124229 AddSecurityScansCreatedAtIndex: migrated (0.0220s) ==========

Index creation takes about 20 seconds in #database-lab so I think it's safe to keep it in regular migration

Query explains

• Old query
• New query

Technically we can improve more by adding index to ci_builds but it's extremely big table and adding indexes there takes really lots of time (around 14h in #database-lab). So I feel like the effort and index costs doesn't worth it compared to 1 monthly query improvement. New query returns small number of records so filtering out shouldn't be a dramatic loss of performance.

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

Related to #335190 (closed)