Use new `--release-notes-file` in release process

[thesuperzapper]

The PR https://github.com/helm/chart-releaser/pull/137 added support to generate the GitHub release notes from a template string (--release-notes-file) rather than the description in Chart.yaml.

We should use this so that we don't have to manually update the description under releases each time.

[aabouzaid]

@thesuperzapper I believe that you want to use --release-notes-file (which was introduced in helm/chart-releaser#137 PR) not --release-name-template :-)

[thesuperzapper]

@aabouzaid thanks, fixed.

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