Use strickter regexp to match milestones

Current regexp gives the wrong milestone, and it fails to update retros.

irb(main):022:0> ReleaseCollection.new(api_client: read_client).previous
=>
{"id"=>4685895,
 "iid"=>115,
 "group_id"=>9970,
 "title"=>"Git 2.47",
 "description"=>"",
 "state"=>"active",
 "created_at"=>"2024-08-08T18:11:51.148Z",
 "updated_at"=>"2024-08-08T18:11:51.148Z",
 "due_date"=>"2024-10-08",
 "start_date"=>"2024-07-30",
 "expired"=>false,
 "web_url"=>"https://gitlab.com/groups/gitlab-org/-/milestones/115"}