ci: automate release notification

[barmac]

Related to #4689

Proposed Changes

As a supplement to #4768, this automates the slack message sent to support.
This is how the message will look like when the PR is merged:

The message will be sent after the release (not RC) is built.

Checklist

To ensure you provided everything we need to look at your PR:

• Brief textual description of the changes present
• Visual demo attached
• Steps to try out present, i.e. using the @bpmn-io/sr tool
• Related issue linked via Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}

[nikku]

Great stuff, let's try this.

What I'd consider: We tag the current release manager, so they are aware of the communication happening (can double check, and check the release issue box).

[barmac]

I will add it.

[barmac]

Added as part of the message:

[fyi] Hi, Desktop Modeler ${{ needs.pre_release.outputs.tag }} release is upcoming. Contact @desktop-modeler-release-manager in case of any questions.