Merge pull request #238502 from l0b0/pr-template-headings

pr-template: Use semantic H2 level headings

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.github
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.github/PULL_REQUEST_TEMPLATE.md
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###### Description of changes
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## Description of changes
<!--
For package updates please link to a changelog or describe changes, this helps your fellow maintainers discover breaking updates.
For new packages please briefly describe the package or provide a link to its homepage.
-->
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###### Things done
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## Things done
<!-- Please check what applies. Note that these are not hard requirements but merely serve as information for reviewers. -->