Fix deprecation warning MD syntax (#171)

The special markdown syntax GitHub is using for warning callouts has changed; this updates it to the new syntax.

README.md

@@ -1,7 +1,7 @@
 [![Code of Conduct](https://img.shields.io/badge/%E2%9D%A4-code%20of%20conduct-blue.svg?style=flat)](https://github.com/edgi-govdata-archiving/overview/blob/master/CONDUCT.md)  [![Project Status Board](https://img.shields.io/badge/✔-Project%20Status%20Board-green.svg?style=flat)](https://github.com/orgs/edgi-govdata-archiving/projects/4)
 
 
-> **Warning**
+> [!WARNING]\
 > **This project is no longer actively maintained.** It may receive security updates, but we are no longer making major changes or improvements. EDGI no longer makes active use of this toolset and it is hard to re-deploy in other contexts.
 >
 > - Looking for tools to monitor websites? Check out our [Awesome Website Change Monitoring](https://github.com/edgi-govdata-archiving/awesome-website-change-monitoring) document or [issue #18](https://github.com/edgi-govdata-archiving/web-monitoring/issues/18), which discusses similar projects. *(This project is most useful if monitoring several thousand pages in bulk, but in most cases, other existing tools will solve your needs faster and cheaper.)*