docs: add upgrade client proposal event (#1596) (#1633)

(cherry picked from commit 41282c7)

Co-authored-by: Carlos Rodriguez <[email protected]>

Author: mergify[bot]

modules/core/spec/06_events.md

@@ -56,7 +56,12 @@ callbacks to IBC applications.
 | update_client_proposal | client_type      | {clientType}      |
 | update_client_proposal | consensus_height | {consensusHeight} |
 
+### UpgradeProposal
 
+| Type                    | Attribute Key   | Attribute Value   |
+|-------------------------|-----------------|-------------------|
+| upgrade_client_proposal | title           | {title}           |
+| upgrade_client_proposal | height          | {height}          |     
 
 ## ICS 03 - Connection