Add a index for vulnerability_state_transitions table

What does this MR do and why?

Describe in detail what your merge request does and why.

This Merge Request adds an index for the vulnerability_id and id on the vulnerability_state_transitions table.

This index would be beneficial for us because:

1. We will most likely want to present the state transition from the latest one to the first one (ORDER BY created_at DESC)
2. The same index would be used for queries like SELECT * FROM vulnerability_state_transitions WHERE vulnerability_in (1,2,3,...,n);

More context can be found in this comment.

The original suggestion of using the created_at was replaced by the id because of this comment and this guideline.

This Merge Request is related to issues #363049 (closed) and #325700 (closed).

Migration Up

main: == 20220530170915 AddIndexForVulnerabilityStateTransition: migrating ==========
main: -- transaction_open?()
main:    -> 0.0000s
main: -- index_exists?(:vulnerability_state_transitions, [:vulnerability_id, :id], {:name=>"index_vulnerability_state_transitions_id_and_vulnerability_id", :algorithm=>:concurrently})
main:    -> 0.0086s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0004s
main: -- add_index(:vulnerability_state_transitions, [:vulnerability_id, :id], {:name=>"index_vulnerability_state_transitions_id_and_vulnerability_id", :algorithm=>:concurrently})
main:    -> 0.0032s
main: -- execute("RESET statement_timeout")
main:    -> 0.0004s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- indexes(:vulnerability_state_transitions)
main:    -> 0.0031s
main: -- remove_index(:vulnerability_state_transitions, {:algorithm=>:concurrently, :name=>"index_vulnerability_state_transitions_on_vulnerability_id"})
main:    -> 0.0032s
main: == 20220530170915 AddIndexForVulnerabilityStateTransition: migrated (0.0309s) =

Migration Down

main: == 20220530170915 AddIndexForVulnerabilityStateTransition: reverting ==========
main: -- transaction_open?()
main:    -> 0.0000s
main: -- indexes(:vulnerability_state_transitions)
main:    -> 0.0052s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- remove_index(:vulnerability_state_transitions, {:algorithm=>:concurrently, :name=>"index_vulnerability_state_transitions_id_and_vulnerability_id"})
main:    -> 0.0028s
main: -- execute("RESET statement_timeout")
main:    -> 0.0004s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- index_exists?(:vulnerability_state_transitions, [:vulnerability_id], {:name=>"index_vulnerability_state_transitions_on_vulnerability_id", :algorithm=>:concurrently})
main:    -> 0.0013s
main: -- add_index(:vulnerability_state_transitions, [:vulnerability_id], {:name=>"index_vulnerability_state_transitions_on_vulnerability_id", :algorithm=>:concurrently})
main:    -> 0.0020s
main: == 20220530170915 AddIndexForVulnerabilityStateTransition: reverted (0.0222s) =

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