[Brave News]: Feed Compositon Improvements Tracking Issue

[LorenzoMinto]

### Tasks
- [ ] https://github.com/brave/brave-browser/issues/36993
- [ ] https://github.com/brave/brave-browser/issues/35969
- [x] [Brave News]: Pull topics for GB locale
- [ ] https://github.com/brave/brave-browser/issues/38515
- [ ] https://github.com/brave/brave-browser/issues/37820
- [ ] [Brave News]: Limit articles in topic clusters to 4
- [ ] https://github.com/brave/brave-browser/issues/36137
- [ ] https://github.com/brave/brave-browser/issues/35659
- [ ] [Brave News]: adjust sampling probabilities for content groups