.github: add CL client to issue template (ethereum#25814)

Author: s1na

.github/ISSUE_TEMPLATE/bug.md

@@ -9,6 +9,7 @@ assignees: ''
 #### System information
 
 Geth version: `geth version`
+CL client & version: e.g. lighthouse/nimbus/prysm@v1.0.0
 OS & Version: Windows/Linux/OSX
 Commit hash : (if `develop`)
 
@@ -27,4 +28,4 @@ Commit hash : (if `develop`)
 [backtrace]
 ````
 
-When submitting logs: please submit them as text and not screenshots.
+When submitting logs: please submit them as text and not screenshots.