Add `comment` column to `vulnerability_state_transitions` table

Why are we doing this work

Marcos promptly identified the lack of comment column in the vulnerability_state_transitions table which would prevent us from migrating the comments present on Vulnerabilities::Feedback models.

Implementation plan

• Create a migration that adds comment:string column
• Create a migration that adds a text limit to comment

See https://docs.gitlab.com/ee/development/database/strings_and_the_text_data_type.html in case the refinement gets stale