Include the app changelog in the app chart release changelog

[lemeurherve]

Currently there is no changelog associated to our helm charts releases.

It would be nice to retrieve and include the upstream changelog.

Ex: for https://github.com/jenkins-infra/helm-charts/releases/tag/accountapp-0.1.16 include the changelog from https://github.com/jenkins-infra/account-app/releases/tag/0.1.6

[lemeurherve]

maybe using the --release-note-file parameter from https://github.com/helm/chart-releaser#create-github-releases-from-helm-chart-packages, with the output of the updatecli changelog section as last_release.txt in the chart folder?

[lemeurherve]

Resolved by #957

[lemeurherve]

Reopening as the changelog introduced by #957 is just the last commit subject in this repo, not the upstream release notes.