Mattermost-server: Set required server version automatically for plugin releases

[mattermod]

When a plugin is released using the circleci orb, we note the required server version in the changelog. This should be automated. The build tool in the plugin template can be used to extract the min server version from the manifest. See https://github.com/mattermost/mattermost-plugin-github/releases/tag/v0.12.0 for an example release note.

Changes to the starter template might be needed.

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JIRA: https://mattermost.atlassian.net/browse/MM-19849

[RajatVaryani]

@hanzei I can also contribute to this as I am already working on https://github.com/mattermost/mattermost-server/issues/13587. Please assign it to me.