Create index on `security_findings`(`scan_id`, `id`)

What does this MR do and why?

This index will be used by the keyset paginator to iterate through security_findings records filtered by scanner_id which times out without this index(#357422 (closed)).

Extracted from Mark stale `security_scans` as `purged` (!82711 - merged) as introducing the index will be faster than merging the entire MR which will also be useful to fix other parts of the software.

Database review

Important Note: The following DDL change runs on a huge table with almost 1B records.

This index will be utilized by queires like this

SELECT
    "security_findings".*
FROM
    "security_findings"
WHERE
    "security_findings"."scan_id" BETWEEN 1 AND 10000
    AND (("security_findings"."scan_id",
            "security_findings"."id") > (1020,
            21852))
ORDER BY
    "security_findings"."scan_id" ASC,
    "security_findings"."id" ASC
LIMIT 10

rake db:migrate:up

== 20220311010352 CreateScanIdAndIdIndexOnSecurityFindings: migrating =========
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:security_findings, [:scan_id, :id], {:name=>"index_security_findings_on_scan_id_and_id", :algorithm=>:concurrently})
   -> 0.0058s
-- execute("SET statement_timeout TO 0")
   -> 0.0004s
-- add_index(:security_findings, [:scan_id, :id], {:name=>"index_security_findings_on_scan_id_and_id", :algorithm=>:concurrently})
   -> 0.0063s
-- execute("RESET statement_timeout")
   -> 0.0004s
== 20220311010352 CreateScanIdAndIdIndexOnSecurityFindings: migrated (0.0224s)

rake db:migrate:down

== 20220311010352 CreateScanIdAndIdIndexOnSecurityFindings: reverting =========
-- transaction_open?()
   -> 0.0000s
-- indexes(:security_findings)
   -> 0.0051s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:security_findings, {:algorithm=>:concurrently, :name=>"index_security_findings_on_scan_id_and_id"})
   -> 0.0044s
-- execute("RESET statement_timeout")
   -> 0.0004s
== 20220311010352 CreateScanIdAndIdIndexOnSecurityFindings: reverted (0.0188s)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.