ISSUE_TEMPLATE: avoid using maintainer usernames as headings (#387825)

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.github/ISSUE_TEMPLATE/06_update_request.yml
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Please mention the people who are in the **Maintainers** list of the offending package. This is done by by searching for the package on the [NixOS Package Search](https://search.nixos.org/packages) and mentioning the people listed under **Maintainers** by prefixing their GitHub usernames with an '@' character. Please add the mentions above the `---` characters in the template below.
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---
**Note for maintainers:** Please tag this issue in your pull request description. (i.e. `Resolves #ISSUE`.)
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.github/ISSUE_TEMPLATE/07_module_request.yml
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Please mention the people who are in the **Maintainers** list of the offending package. This is done by by searching for the package on the [NixOS Package Search](https://search.nixos.org/packages) and mentioning the people listed under **Maintainers** by prefixing their GitHub usernames with an '@' character. Please add the mentions above the `---` characters in the template below.
value: |
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---
**Note for maintainers:** Please tag this issue in your pull request description. (i.e. `Resolves #ISSUE`.)
+1
.github/ISSUE_TEMPLATE/08_backport_request.yml
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Please mention the people who are in the **Maintainers** list of the offending package. This is done by by searching for the package on the [NixOS Package Search](https://search.nixos.org/packages) and mentioning the people listed under **Maintainers** by prefixing their GitHub usernames with an '@' character. Please add the mentions above the `---` characters in the template below.
value: |
+
---
**Note for maintainers:** Please tag this issue in your pull request description. (i.e. `Resolves #ISSUE`.)