This is happen to me to in my production.
From 2.9.0.beta14 (2b9fa41a6e) to 3.1.0.beta1
Fetching gem metadata from https://rubygems.org/.........
Fetching https://github.com/rails/sprockets
web-push-3.0.0 requires ruby version >= 3.0, which is incompatible with the
current version, 2.7.6
Docker Manager: FAILED TO UPGRADE
#<RuntimeError: RuntimeError>
/var/www/discourse/plugins/docker_manager/lib/docker_manager/upgrader.rb:209:in `run'
/var/www/discourse/plugins/docker_manager/lib/docker_manager/upgrader.rb:93:in `upgrade'
/var/www/discourse/plugins/docker_manager/scripts/docker_manager_upgrade.rb:19:in `block in <main>'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/activesupport-7.0.3.1/lib/active_support/fork_tracker.rb:20:in `block in fork'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/activesupport-7.0.3.1/lib/active_support/fork_tracker.rb:18:in `fork'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/activesupport-7.0.3.1/lib/active_support/fork_tracker.rb:18:in `fork'
/var/www/discourse/plugins/docker_manager/scripts/docker_manager_upgrade.rb:6:in `<main>'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/railties-7.0.3.1/lib/rails/commands/runner/runner_command.rb:43:in `load'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/railties-7.0.3.1/lib/rails/commands/runner/runner_command.rb:43:in `perform'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/thor-1.2.1/lib/thor/command.rb:27:in `run'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/thor-1.2.1/lib/thor/invocation.rb:127:in `invoke_command'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/thor-1.2.1/lib/thor.rb:392:in `dispatch'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/railties-7.0.3.1/lib/rails/command/base.rb:87:in `perform'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/railties-7.0.3.1/lib/rails/command.rb:48:in `invoke'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/railties-7.0.3.1/lib/rails/commands.rb:18:in `<main>'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/bootsnap-1.15.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:32:in `require'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/bootsnap-1.15.0/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:32:in `require'
bin/rails:18:in `<main>'
Spinning up 1 Unicorn worker(s) that were stopped initially
Solved with 
cd /var/discourse
git pull
./launcher rebuild app
thanks