Add "issue severity" back to GitHub bug template (#23838)

Adds back the "Issue severity" dropdown to the bug template so that Ray users can have a way of reporting UX problems. Made some changes to try to streamline issue reporting:

- made this field optional
- moved the field to be last
- slightly changed some of the wording

.github/ISSUE_TEMPLATE/bug-report.yml

@@ -47,3 +47,16 @@ body:
  until the repro instruction is updated.
  validations:
  required: true
+
+ - type: dropdown
+ attributes:
+ label: Issue Severity
+ description: |
+ How does this issue affect your experience as a Ray user?
+ multiple: false
+ options:
+ - "Low: It annoys or frustrates me."
+ - "Medium: It is a significant difficulty but I can work around it."
+ - "High: It blocks me from completing my task."
+ validations:
+ required: false