Create pull_request_template.md

Creating a pull request template to ensure specific information is documented and calling out the versions of GHES that are supported by backup-utils so that appropriate testing can be done against those versions

Author: shcorbett

.github/pull_request_template.md

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+### Description
+<!--
+[Please fill out a brief description of the change being made]
+-->
+### Testing
+<!--
+[Please add testing done as part of this change.] 
+-->
+<!-- Keep in mind that for backup-utils the following applies:
+- Backup-util [current version] will support
+   - GHES [current version]
+   - GHES [current version -1]
+   - GHES [current version -2]
+- Any changes that are made to backup-utils will also need to be supported on those GHES versions above (n-2)
+- Please make sure those versions are tested against for this change 
+-->
+
+### Owernship
+<!-- [Add any relevants owners for this change]
+-->
+
+### Related Links
+<!-- [Please add any related links/issues to this PR]
+-->