upgrade to SDK 0.15.5 (frontend + backend) (#49)

## Motivation

apply fix for WrongRound errors + normal process

## Proposal

upgrade to SDK 0.15.5 (frontend + backend)

## Test Plan

CI + tested manually

Author: ma2bd

.github/workflows/main.yml

@@ -29,12 +29,12 @@ jobs:
       - name: 'Install linera-storage-service'
         uses: taiki-e/cache-cargo-install-action@v2
         with:
-          tool: [email protected].3
+          tool: [email protected].5
 
       - name: 'Install linera-service'
         uses: taiki-e/cache-cargo-install-action@v2
         with:
-          tool: [email protected].3
+          tool: [email protected].5
 
       - run: |
           ln -sf /opt/hostedtoolcache/linera-storage-service/bin/linera-storage-server /opt/hostedtoolcache/linera-service/bin/