fix: remove response template context (#285)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Dependency Update**
  - Upgraded `@eggjs/koa` package from version `^2.20.3` to `^2.20.6`

- **Type System Refinements**
  - Simplified `Response` class type definition
- Updated `MiddlewareFunc` type to provide more flexible generic type
handling

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

package.json

@@ -38,7 +38,7 @@
   },
   "homepage": "https://github.com/eggjs/core#readme",
   "dependencies": {
-    "@eggjs/koa": "^2.20.3",
+    "@eggjs/koa": "^2.20.6",
     "@eggjs/router": "^3.0.5",
     "@eggjs/utils": "^4.1.5",
     "egg-logger": "^3.5.0",

src/egg.ts

@@ -50,7 +50,7 @@ export class Request extends KoaRequest {
   declare response: Response;
 }
 
-export class Response extends KoaResponse<Context> {
+export class Response extends KoaResponse {
   declare app: EggCore;
   declare request: Request;
 }
@@ -63,7 +63,7 @@ export class Context extends KoaContext {
 }
 
 // export @eggjs/core types
-export type MiddlewareFunc<T extends Context = Context> = KoaMiddlewareFunc<T>;
+export type MiddlewareFunc<T = Context> = KoaMiddlewareFunc<T>;
 
 export class EggCore extends KoaApplication {
   options: EggCoreOptions;