Add GitHub Issue and PR templates

.github/ISSUE_TEMPLATE.md

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+<!-- 
+Thanks for raising a Spring Boot issue. Please provide a brief description of your problem along with the version of Spring Boot that you are using. If possible, please also consider putting together a sample application that reproduces the issue.
+--> 

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- 
+Thanks for contributing to Spring Boot. Please provide a brief description of your pull-request and reference any related issue numbers (prefix references with #).
+--> 
+
+<!-- Please also confirm that you have signed the CLA by put an [X] in the box below: -->
+- [ ] I have signed the CLA