fix(api): grpc content type matcher all grpc types (#8285)

# Which Problems Are Solved

ZITADEL returned a 404 Unimplemented error if the client sent
'application/grpc+proto' or 'application/grpc+json' which are both valid
content types.

# How the Problems Are Solved

changed the header matcher to regexp

# Additional Context

Problem occured in
https://github.com/zitadel/typescript/tree/grpc-transport

(cherry picked from commit aa273ad000)

internal/api/api.go

@@ -180,7 +180,7 @@ func (a *API) RouteGRPC() {
 		Name("grpc")
 	http2Route.
 		Methods(http.MethodPost).
-		Headers("Content-Type", "application/grpc").
+		HeadersRegexp(http_util.ContentType, `application\/grpc(\+proto|\+json)?`).
 		Handler(a.grpcServer)
 
 	a.routeGRPCWeb()