Notifications on new tags
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### Release notes
<!-- What is the problem and solution you're proposing? This content sets the overall vision for the feature and serves as the release notes that will populate in various places, including the [release post blog](https://about.gitlab.com/releases/categories/releases/) and [Gitlab project releases](https://gitlab.com/gitlab-org/gitlab/-/releases). " -->
### Problem to solve
<!-- What is the user problem you are trying to solve with this issue? -->
To subscribe to email notifications when a project has a new tag created
### Proposal
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Add `New Tag` to the custom notification options
### Intended users
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Product Manager, DevOps Engineer
### Feature Usage Metrics
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This is difficult to answer. I work with some teams that do not use the release feature, and instead create new tags on their projects. If I were to be able to select to subscribe to new tags, I would be able to know that my team will need to support a deployment. I'm not sure how to track usage of the feature though.
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