Update issue templates [skip-ci] (root-project#6330)

[github] Customize issue templates (NFC).

Co-authored-by: Stephan Hageboeck <[email protected]>

Author: Axel-Naumann

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to get an issue fixed
+title: ''
+labels: bug
+---
+
+## Describe the bug
+<!--
+A clear and concise description of what the wrong behavior is.
+-->
+
+## Expected behavior
+<!--
+A clear and concise description of what you expected to happen.
+-->
+
+## To Reproduce
+<!--
+Steps to reproduce the behavior:
+1. Your code that triggers the issue: at least a part; ideally something we can run ourselves.
+2. Don't forget to attach the required input files!
+3. How to run your code and / or build it, e.g. `root myMacro.C`, ...
+-->
+
+## Setup
+<!--
+1. ROOT version
+2. Operating system
+3. How you obtained ROOT, such as `dnf install` / binary download / you built it yourself.
+-->
+
+## Additional context
+<!--
+Add any other context about the problem here.
+-->

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Propose a new feature for ROOT
+title: ''
+labels: new feature
+---
+
+## Is your feature request related to a problem? Please describe.
+<!--
+A clear and concise description of what the problem is. E.g "I always have to [...] when I want to [...]"
+-->
+
+## Describe the solution you'd like
+<!--
+A clear and concise description of what you want to happen.
+-->
+
+## Describe alternatives you've considered
+<!--
+Can you think of alternative solutions or features?
+-->
+
+## Additional context
+<!--
+Add any other context or screenshots about the feature requested here.
+-->

.github/ISSUE_TEMPLATE/improvement_report.md

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+---
+name: Improvement
+about: Suggest something that could be improved.
+title: ''
+labels: improvement
+---
+
+## Explain what you would like to see improved
+<!--
+Explain what isn't as good as it could be and why
+-->
+
+## Optional: share how it could be improved
+<!--
+If you already have an idea what we could improve, then please tell us.
+-->
+
+## To Reproduce
+<!--
+Steps to reproduce the behavior:
+1. Your code that triggers the issue: at least a part; ideally something we can run ourselves.
+2. Don't forget to attach the required input files!
+3. How to run your code: build it / `root myMacro.C` / ...
+-->
+
+## Setup
+<!--
+1. ROOT version
+2. Operating system
+3. How you obtained ROOT, such as `dnf install` / binary download / you built it yourself.
+-->
+
+## Additional context
+<!--
+Add any other context about the problem here.
+-->