🔥 removed old issue template

.github/ISSUE_TEMPLATE.md

-**Bug Report**
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-- What is the issue you have?
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-- Please describe the steps to reproduce the issue. Can you provide a small but working code example?
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-- What is the expected behavior?
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-- And what is the actual behavior instead?
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-- Which compiler and operating system are you using? Is it a [supported compiler](https://github.com/nlohmann/json#supported-compilers)?
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-- Did you use a released version of the library or the version from the `develop` branch?
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-- If you experience a compilation error: can you [compile and run the unit tests](https://github.com/nlohmann/json#execute-unit-tests)?
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-**Feature Request**
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-- Describe the feature in as much detail as possible.
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-- Include sample usage where appropriate.