Resolve "Validate against Vulnerabilities::StateTransition to_state and from_state being the same to prevent redundant record creation"

What does this MR do and why?

To prevent the creation of redundant Vulnerability State Transition records where there is no actual state change occurring, this MR implements a database level check constraint to ensure that the to_state and from_state fields of a transition record differ.

How to set up and validate locally

Assuming an existing vulnerability state transition exists in your db, open a rails console and run the following:

Vulnerabilities::StateTransition.create(Vulnerabilities::StateTransition.last.attributes.except('id').merge('to_state' => :dismissed, 'from_state' => :dismissed))

This should result in the following failure:

ActiveRecord::StatementInvalid: PG::CheckViolation: ERROR:  new row for relation "vulnerability_state_transitions" violates check constraint "check_d1ca8ec043"
DETAIL:  Failing row contains (2, 128, 2, 2, 2022-10-25 11:33:45.946096+00, 2022-10-25 11:33:45.946096+00, null, null, null).

from /data/cache/bundle-2.7.6/ruby/2.7.0/gems/activerecord-6.1.6.1/lib/active_record/connection_adapters/postgresql_adapter.rb:672:in `exec_params'
Caused by PG::CheckViolation: ERROR:  new row for relation "vulnerability_state_transitions" violates check constraint "check_d1ca8ec043"
DETAIL:  Failing row contains (2, 128, 2, 2, 2022-10-25 11:33:45.946096+00, 2022-10-25 11:33:45.946096+00, null, null, null).

from /data/cache/bundle-2.7.6/ruby/2.7.0/gems/activerecord-6.1.6.1/lib/active_record/connection_adapters/postgresql_adapter.rb:672:in `exec_params'

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.

Related to #375497 (closed)