fix: use setup-go@v3 (latest available on gitea.com mirror)

The gitea.com/actions/setup-go mirror only has tags up to v3.
v3 supports go-version-file which is all we need.

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

.gitea/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: https://gitea.com/actions/checkout@v4
-      - uses: https://gitea.com/actions/setup-go@v5
+      - uses: https://gitea.com/actions/setup-go@v3
         with:
           go-version-file: go.mod
       - run: go build ./...
@@ -20,7 +20,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: https://gitea.com/actions/checkout@v4
-      - uses: https://gitea.com/actions/setup-go@v5
+      - uses: https://gitea.com/actions/setup-go@v3
         with:
           go-version-file: go.mod
       - run: go test ./...
@@ -29,7 +29,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: https://gitea.com/actions/checkout@v4
-      - uses: https://gitea.com/actions/setup-go@v5
+      - uses: https://gitea.com/actions/setup-go@v3
         with:
           go-version-file: go.mod
       - run: go vet ./...