.github: removing "Priorities" heading

Where this was a markdown heading, it sometimes had the wrong weight
so it appeared as if it was a subheading of the previous section, and
some people feel the word "Priorities" creates misleading
expectations.

Link: https://github.com/NixOS/nixpkgs/pull/203969/files#r1037761779

.github/ISSUE_TEMPLATE/bug_report.md

@@ -40,7 +40,7 @@ Please run `nix-shell -p nix-info --run "nix-info -m"` and paste the result.
 output here
 ```
 
-### Priorities
+---
 
 Add a :+1: [reaction] to [issues you find important].
 

.github/ISSUE_TEMPLATE/build_failure.md

@@ -38,7 +38,7 @@ Please run `nix-shell -p nix-info --run "nix-info -m"` and paste the result.
 output here
 ```
 
-### Priorities
+---
 
 Add a :+1: [reaction] to [issues you find important].
 

.github/ISSUE_TEMPLATE/missing_documentation.md

@@ -30,7 +30,7 @@ assignees: ''
 [open documentation issues]: https://github.com/NixOS/nixpkgs/issues?q=is%3Aissue+is%3Aopen+label%3A%229.needs%3A+documentation%22
 [open documentation pull requests]: https://github.com/NixOS/nixpkgs/pulls?q=is%3Aopen+is%3Apr+label%3A%228.has%3A+documentation%22%2C%226.topic%3A+documentation%22
 
-### Priorities
+---
 
 Add a :+1: [reaction] to [issues you find important].
 

.github/ISSUE_TEMPLATE/out_of_date_package_report.md

@@ -27,7 +27,7 @@ There's a high chance that you'll have the new version right away while helping
 
 Note for maintainers: Please tag this issue in your PR.
 
-**Priorities**
+---
 
 Add a :+1: [reaction] to [issues you find important].
 

.github/ISSUE_TEMPLATE/packaging_request.md

@@ -18,7 +18,7 @@ assignees: ''
 * license: mit, bsd, gpl2+ , ...
 * platforms: unix, linux, darwin, ...
 
-**Priorities**
+---
 
 Add a :+1: [reaction] to [issues you find important].
 

.github/ISSUE_TEMPLATE/unreproducible_package.md

@@ -86,7 +86,7 @@ nix log $(nix path-info --derivation nixpkgs#<package>)
 (please share the relevant fragment of the diffoscope output here, and any
 additional analysis you may have done)
 
-### Priorities
+---
 
 Add a :+1: [reaction] to [issues you find important].
 

.github/PULL_REQUEST_TEMPLATE.md

@@ -41,7 +41,7 @@ List of open PRs: https://github.com/NixOS/nixpkgs/pulls
 Reviewing guidelines: https://nixos.org/manual/nixpkgs/unstable/#chap-reviewing-contributions
 -->
 
-### Priorities
+---
 
 Add a :+1: [reaction] to [pull requests you find important].