Document that JUnit 4 is in maintenance mode (#1786)

* Add to README
* Add to issue template
* Add to PR template

Author: marcphilipp

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,11 @@
+---
+name: Report a critical bug
+about: Please only report critical bugs since the project is in maintenance mode
+type: Bug
+---
+
+> [!IMPORTANT]  
+> **JUnit 4 is in maintenance mode.**
+>
+> At this point, only critical bugs and security issues will be fixed.<br>
+> All other issues and PRs will therefore be declined.

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false

.github/PULL_REQUEST_TEMPLATE.md

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+> [!IMPORTANT]  
+> **JUnit 4 is in maintenance mode.**
+>
+> At this point, only critical bugs and security issues will be fixed.<br>
+> All other issues and PRs will therefore be declined.
+
+## Overview
+
+<!-- Please describe your changes here and list any open questions you might have. -->

README.md

@@ -1,6 +1,12 @@
 # JUnit 4
 JUnit is a simple framework to write repeatable tests. It is an instance of the xUnit architecture for unit testing frameworks.
 
+> [!IMPORTANT]  
+> **JUnit 4 is in maintenance mode.**
+>
+> At this point, only critical bugs and security issues will be fixed.<br>
+> All other issues and PRs will therefore be declined.
+
 For more information, please visit:
 * [Wiki](https://github.com/junit-team/junit4/wiki)
 * [Download and Install guide](https://github.com/junit-team/junit4/wiki/Download-and-Install)