Add `scala-steward` To All WebX Repos

[JamieB-gu]

### Tasks
- [ ] https://github.com/guardian/frontend/issues/26781
- [ ] https://github.com/guardian/skimlinks-lambda/issues/16
- [ ] https://github.com/guardian/frontend-lambda/issues/59
- [ ] https://github.com/guardian/platform/issues/1564
- [ ] https://github.com/guardian/top-mentions-aggregator/issues/40