Rename vulnerability_finding's fingerprints to signatures

What does this MR do?

Avoids internal naming conflicts with existing fingerprint security attributes, such as location_fingerprint.

Since the vulnerability_finding_fingerprints table does not have any Production data in it, we have decided to drop that table and create the vulnerability_finding_signatures table.

Relates to #326231 (closed) and https://gitlab.com/groups/gitlab-org/-/epics/4690

Database migration

Down Migration - ran locally:

== 20210330130420 DropFindingFingerprintTable: reverting ======================
-- create_table(:vulnerability_finding_fingerprints)
   -> 0.0146s
== 20210330130420 DropFindingFingerprintTable: reverted (0.0214s) =============

== 20210329191850 AddFindingSignatureTable: reverting =========================
-- drop_table(:vulnerability_finding_signatures)
   -> 0.0018s
== 20210329191850 AddFindingSignatureTable: reverted (0.0061s) ================

Up Migration - ran locally:

== 20210329191850 AddFindingSignatureTable: migrating =========================
-- create_table(:vulnerability_finding_signatures)
   -> 0.0149s
== 20210329191850 AddFindingSignatureTable: migrated (0.0207s) ================

== 20210330130420 DropFindingFingerprintTable: migrating ======================
-- drop_table(:vulnerability_finding_fingerprints)
   -> 0.0017s
== 20210330130420 DropFindingFingerprintTable: migrated (0.0056s) =============

See this thread for results of running the migration against gitlab.com.

Does this MR meet the acceptance criteria?

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