Raise Gitaly default timeout to 600 s instead of 55

Users are unable to fork large repositories (e.g. www-gitlab-com) because of #1187 (closed).

Until we roll out gitlab-org/gitlab!17719 (merged), I think we should raise this timeout from 55 to 600 seconds based on the data. Very few services use this timeout, but the first two are done in background jobs:

$ git grep default_timeout
lib/gitlab/gitaly_client.rb:    def self.call(storage, service, rpc, request, remote_storage: nil, timeout: default_timeout)
lib/gitlab/gitaly_client.rb:    def self.default_timeout
lib/gitlab/gitaly_client/repository_service.rb:          GitalyClient.default_timeout
lib/gitlab/gitaly_client/repository_service.rb:          GitalyClient.default_timeout
lib/gitlab/gitaly_client/repository_service.rb:        response = GitalyClient.call(@storage, :repository_service, :search_files_by_content, request, timeout: GitalyClient.default_timeout)
spec/lib/gitlab/gitaly_client_spec.rb:        expect(described_class.default_timeout).to be(55)