Make MatrixRTC encryption key types narrower for TS 5.9 compatibility (#5117)
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-5-9.html#libdts-changes
TypeScript 5.9 changes some things about the ArrayBuffer type and makes a number of DOM types, including the subtle crypto APIs, require a narrower buffer type as their input. For example if you wanted to use crypto.subtle.importKey to convert a MatrixRTC encryption key buffer given by matrix-js-sdk to a CryptoKey, you would run into a type error with TS 5.9. Specifying the type parameter of Uint8Array everywhere around the MatrixRTC files fixes this breakage.