Bump actions/setup-go from v5 to v6 in all workflows

Resolves Node.js 20 deprecation warning. v6 uses Node.js 24.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ako

.github/workflows/nightly.yml

@@ -15,7 +15,7 @@ jobs:
     steps:
       - uses: actions/checkout@v5
 
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version: '1.26'
 
@@ -40,7 +40,7 @@ jobs:
         with:
           fetch-depth: 0
 
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version: '1.26'
 

.github/workflows/push-test.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v5
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version: '1.26'
       - name: Build

.github/workflows/release.yml

@@ -15,7 +15,7 @@ jobs:
       - uses: actions/checkout@v5
         with:
           fetch-depth: 0
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version: '1.26'
       - name: Build release binaries