Migrate to golangci-lint v2

`golangci-lint-action` Node version is too old, so actions give a warning. It needs to be upgraded to at least `@v7`, but this requires golangci-lint v2.

Timeline:
- June 2, 2026: needs `ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION=true` to keep working
- Sept 16, 2026: doesn't work anymore