ISSUE_TEMPLATE.md: clear the blank issue template

This changes the blank issue template to a simple HTML comment telling
users to use the forms instead, while still allowing experienced
contributors to create free-form issues.

Signed-off-by: Fernando Rodrigues <alpha@sigmasquadron.net>

Changed files
+6 -11
.github
+6 -11
.github/ISSUE_TEMPLATE.md
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## Issue description
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### Steps to reproduce
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## Technical details
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<!-- Please insert the output of running `nix-shell -p nix-info --run "nix-info -m"` below this line -->
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<!--
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Please note: This blank issue template is meant for extraordinary issues
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that do not fit the templates. Unless you know your issue is relevant to
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Nixpkgs and requires the free-form blank issue, please use the issue
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templates instead.
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-->