Add start_date to issues table

What does this MR do and why?

First step of #365399 (closed)

Adds start_date to the issues table. We have two models that use the issues, Issue and WorkItem < Issue. Adding the validation in the Issue model as adding it to WorkItem (where it is relevant for a feature), would allow creating/updating issues with invalid start/due dates thus making some work items invalid.

Migration Output

UP

$ bin/rails db:migrate
main: == 20220720210446 AddStartDateToIssuesTable: migrating ========================
main: -- add_column(:issues, :start_date, :date)
main:    -> 0.0030s
main: == 20220720210446 AddStartDateToIssuesTable: migrated (0.0037s) ===============

ci: == 20220720210446 AddStartDateToIssuesTable: migrating ========================
ci: -- add_column(:issues, :start_date, :date)
ci:    -> 0.0033s
ci: == 20220720210446 AddStartDateToIssuesTable: migrated (0.0035s) ===============

DOWN

$ bin/rails db:rollback:ci
ci: == 20220720210446 AddStartDateToIssuesTable: reverting ========================
ci: -- remove_column(:issues, :start_date, :date)
ci:    -> 0.0032s
ci: == 20220720210446 AddStartDateToIssuesTable: reverted (0.0114s) ===============

$ bin/rails db:rollback:main
main: == 20220720210446 AddStartDateToIssuesTable: reverting ========================
main: -- remove_column(:issues, :start_date, :date)
main:    -> 0.0027s
main: == 20220720210446 AddStartDateToIssuesTable: reverted (0.0047s) ===============

MR acceptance checklist

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Related to #365399 (closed)