Update the issue template for new way of getting server version (#6051)

cf #4878

.github/ISSUE_TEMPLATE/BUG_REPORT.md

@@ -7,7 +7,7 @@ about: Create a report to help us improve
 <!--
 
 **IF YOU HAVE SUPPORT QUESTIONS ABOUT RUNNING OR CONFIGURING YOUR OWN HOME SERVER**:
-You will likely get better support more quickly if you ask in ** #matrix:matrix.org ** ;)
+You will likely get better support more quickly if you ask in ** #synapse:matrix.org ** ;)
 
 
 This is a bug report template. By following the instructions below and
@@ -50,9 +50,13 @@ If not matrix.org:
 
 <!--
  What version of Synapse is running?
-You can find the Synapse version by inspecting the server headers (replace matrix.org with
-your own homeserver domain):
-$ curl -v https://matrix.org/_matrix/client/versions 2>&1 | grep "Server:"
+
+You can find the Synapse version with this command:
+
+$ curl http://localhost:8008/_synapse/admin/v1/server_version
+
+(You may need to replace `localhost:8008` if Synapse is not configured to
+listen on that port.)
 -->
 - **Version**: