Add GitHub Issue templates

[nosan]

If you think it's a good idea, it might be worth considering adding templates for PRs.

[nosan]

@mhalbritter I reviewed the issue page at https://github.com/spring-io/start.spring.io/issues/new/choose, and I noticed it includes a Report of a security vulnerability option, but the repository itself does not have a SECURITY.md.

Is it possible to do something similar in the Spring Boot repository? If so, I could remove the link from contact_links

[bclozel]

We recently applied this in Spring Framework because we had several security reports made public as issues. I'm not sure we have the same problem in the Spring Boot repository at this point.

Also about the issue type, its title is "Issue" which renders as "Issue: Issue" in the GitHub UI. See https://github.com/spring-projects/spring-framework/issues/new?assignees=&labels=&projects=&template=issue.md&title=

[nosan]

Thank you, @snicoll