Add VSA to Plan retro

Problem to solve

https://gitlab.com/gitlab-org/plan/-/issues/753+

Result

This MR introduces Value Stream Analytics to the Plan retro.

It refactors the current_release to return the milestone object instead of only the title, as the start_date and due_date is required to link to VSA correctly.

Dry run addition on updating the issue description

## Value Stream Analytics

Visualize the key metrics for the release by reviewing the Value Stream Analytics (VSA).

- [VSA for this release]: https://gitlab.com/groups/gitlab-org/-/analytics/value_stream_analytics?value_stream_id=1022&created_after=2022-09-18&created_before=2022-10-17&label_name[]=devops%3A%3Aplan&milestone_title=15.5
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Follow up after this MR

• Refactor the reference to release to be accurate across the project i.e. release => release_title - this requires updating all template references so deemed out of scope here.

release: release['title'],

# to 

release_title: release['title'],

• Add the previous release to the retrospective too i.e. an additional API call via current_release with index: -3 / or return 2 releases from the function (more refactoring but better performance.