Update triage guide to match the new label scheme

Element Web issue labels have been changed to match a new scheme that has been
piloted by the [Synapse
team](https://github.com/matrix-org/synapse/issues/9460). The new triage
guidance here is largely borrowed from the Synapse guide, but with some
Element-specifics to reflect how we use priority, add code areas, etc.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -2,7 +2,7 @@
 name: Bug report
 about: Create a report to help us improve
 title: ''
-labels: bug
+labels: T-Defect
 assignees: ''
 
 ---
@@ -46,7 +46,7 @@ Logs being sent: yes/no
 
 For the web app:
 
-- **Browser**: Chrome, Safari, Firefox? which version?
+- **Browser**: Chrome, Firefox, Safari, Edge? which version?
 - **OS**: Windows, macOS, Ubuntu, Arch Linux, etc?
 - **URL**: develop.element.io / app.element.io / somewhere else? If a private server, what version of Element Web?