Drop Rails 7.0 support

[excid3]

Summary

Rails 7.0 is EOL. Requires rails >= 7.1.0.

• Remove rails-7-0 from Appraisals, delete its gemfile/lock, drop it from all three CI matrices (and the Ruby 3.4/4.0 excludes that only existed for it).
• Remove the four code branches that only served 7.0: serialize coder: keyword check in Event, the perform_all_later shim in EventJob, the HtmlSafeTranslation guard in Translation, and the fixture_path fallback in test_helper.
• Refresh the six remaining lockfiles for the new gemspec dependency.
• Bump the deprecation horizon from "3.0" to "4.0" — the 2.x deprecations (Noticed::Base, deliver_by :database, param/params) are still shipping in 3.x.

Test plan

• standardrb clean
• rails test on main Gemfile, Rails 7.1, 7.2, 8.0, 8.1, main — 152 runs, 0 failures each
• CI matrix (now 5 gemfiles × 3 Rubies minus excludes, × 3 databases)