Update flat-manager-client to 0.5.0

[ryonakano]

Fixes #229

Regarding to Dockerfile, I locally built the image and confirmed the version of flat-manager-client is 0.5.0:

user@elementary-daily:~/work/elementary/flatpak-platform (main +%=)$ docker buildx build -t flatpak-platform .
[+] Building 233.2s (10/10) FINISHED                                                                                           docker:default
 => [internal] load build definition from Dockerfile                                                                                     0.0s
 => => transferring dockerfile: 631B                                                                                                     0.0s
 => [internal] load metadata for docker.io/library/ubuntu:latest                                                                         0.0s
 => [internal] load .dockerignore                                                                                                        0.0s
 => => transferring context: 2B                                                                                                          0.0s
 => CACHED [1/5] FROM docker.io/library/ubuntu:latest                                                                                    0.0s
 => [4/5] ADD https://github.com/flatpak/flat-manager/releases/download/0.5.0/flat-manager-client.arm64 /usr/bin/flat-manager-client    30.0s
 => [2/5] RUN apt-get update &&     apt-get -y install flatpak flatpak-builder python3-aiohttp python3-tenacity python3-gi libostree-  226.6s
 => [3/5] RUN flatpak remote-add --if-not-exists appcenter https://flatpak.elementary.io/repo.flatpakrepo                                1.0s
 => [4/5] ADD https://github.com/flatpak/flat-manager/releases/download/0.5.0/flat-manager-client.arm64 /usr/bin/flat-manager-client     0.0s
 => [5/5] RUN chmod +x /usr/bin/flat-manager-client                                                                                      0.1s
 => exporting to image                                                                                                                   5.5s
 => => exporting layers                                                                                                                  5.5s
 => => writing image sha256:e1e509ae00923e0d24d2bdadab031dbf6ba622c7014a542b275038bb2b793d69                                             0.0s
 => => naming to docker.io/library/flatpak-platform                                                                                      0.0s
user@elementary-daily:~/work/elementary/flatpak-platform (ryonakano/update-flat-manager-client %)$ docker run -it flatpak-platform
root@cb2a66f55caa:/# flat-manager-client --version
flat-manager-client 0.5.0
root@cb2a66f55caa:/#