ci: upgrade golangci-lint-action to v7 and pin v2.5.0

The v6 action with version: latest installs golangci-lint v1.64.8,
which can't parse the v2 schema in .golangci.yml and was itself built
with Go 1.24 — both incompatible with this repo's go.mod (go 1.25.1).
v7 of the action natively supports v2; pinning v2.5.0 matches the
version we run locally.

Author: nmolchanov

.github/workflows/ci.yml

@@ -17,9 +17,9 @@ jobs:
           cache: true
 
       - name: golangci-lint
-        uses: golangci/golangci-lint-action@v6
+        uses: golangci/golangci-lint-action@v7
         with:
-          version: latest
+          version: v2.5.0
 
       - name: Test
         run: go test -race -cover ./...