[code-infra] Adds release announcement to github workflows

[michelengelen]

• [x] I have followed (at least) the PR section of the contributing guide.

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This action will send a message to the #announcements channel on our discord server when there is a new release (events: release:published and release:prereleased).

For this to work we need to add a separator line into the changelog. This will be a comment string sitting just before the detailed changes.

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<!--/ DO_NOT_REMOVE /-->

### Data Grid

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This string needs to stay in order for the script to know where the highlights end. The reason why we are doing this is that Discord does not allow messages to be above 2000 characters.

The message posted will look like this (tested with the v7-beta-0 Changelog):

[mui-bot]

Deploy preview: https://deploy-preview-11867--material-ui-x.netlify.app/

Generated by :no_entry_sign: dangerJS against aca244cd8e8a7e08d4e31ec2366e0ada23390816

[michelengelen]

Nice 👍

I guess we don't use https://github.com/marketplace/actions/github-releases-to-discord to have the highlight extract.

No, we don't, since every action available was either not working or did not have the feature set we wanted.

[github-actions[bot]]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[michelengelen]

Can we move on with this PR or are there any change requests from either of you? @DanailH @joserodolfofreitas @oliviertassinari