Add comment to vulnerability_state_transitions table

What does this MR do and why?

This MR adds comment:text column to vulnerability_state_transitions so we can migrate records off from the vulnerability_feedback table.

Related to #369932 (closed)

Database review

Text limit

I ran the following query:

gitlabhq_production=> SELECT MAX(char_length(comment)) FROM vulnerability_feedback;
 max 
-----
  49
(1 row)

in order to determine the maximum length of what we have in the comment column on vulnerability_feedback table

MR acceptance checklist

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