[KYUUBI #6988] [INFRA] Foward GitHub discussions to ASF mailing list

### Why are the changes needed?

To satisfy the ASF requirements.

> An error occurred while processing the github feature in .asf.yaml:
>
> GitHub discussions can only be enabled if a mailing list target exists for it.

### How was this patch tested?

Review and monitor the master branch after merging.

### Was this patch authored or co-authored using generative AI tooling?

No

Closes #6988 from pan3793/discussion.

Closes #6988

dd788c0 [Cheng Pan] [INFRA] Foward GitHub discussions to ASF mailing list

Authored-by: Cheng Pan <[email protected]>
Signed-off-by: Cheng Pan <[email protected]>

Author: pan3793

Diff for: .asf.yaml

@@ -45,3 +45,4 @@ notifications:
   commits: [email protected]
   issues: [email protected]
   pullrequests: [email protected]
+  discussions: [email protected]