Merge branch '441146-vue3-migration-burndown_chart_spec' into 'master'

Removes direct data manipulation from burndown_chart_spec

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/144241



Merged-by: Eduardo Sanz García <esanz-garcia@gitlab.com>
Approved-by: Marina Mosti <mmosti@gitlab.com>
Approved-by: Eduardo Sanz García <esanz-garcia@gitlab.com>
Reviewed-by: Artur Fedorov <afedorov@gitlab.com>
Reviewed-by: Marina Mosti <mmosti@gitlab.com>
Co-authored-by: Artur Fedorov <afedorov@gitlab.com>