Update standard samples to use ruby 3.0

[wietsevenema]

Description

Ruby 3.0 is now generally available on App Engine standard, this PR updates standard samples.

• appengine/standard-analytics
• appengine/standard-cloud-tasks
• appengine/standard-datastore
• appengine/standard-hello_configs
• appengine/standard-hello_world
• appengine/standard-metadata-server
• appengine/standard-storage

Checklist

• [x] Tests pass
• [x] Lint pass: bundle exec rubocop
• [x] Please merge this PR for me once it is approved.