ISSUE_TEMPLATE: avoid using maintainer usernames as headings

[keysmashes]

#368656 moved the issue templates to YAML; this is awesome, but had the (unintentional?) side-effect of making the low-effort way to tag maintainers use markdown heading markup for the tags, rather than the previously-used horizontal rule below them.

If you tag a maintainer like this:

The result looks like this:

I suspect what was intended was instead this:

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This change is already visible in recent issues, e.g. #387705:

whereas older issues (searching for "note for maintainers" created:<2025-01-13, e.g. #362118) consistently use normal text followed by a horizontal rule:

@SigmaSquadron as the author of #368656, please do shout if this change was deliberate!

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[FliegendeWurst]

There are actually more affected templates, for example the bug report templates. I'll take care of it later, or you can do it.