Drop support for Ruby 3.0 with Rails+Rubygems

`gem update --system` will always attempt to install the latest version
of Rubygems, even if it's not compatible with the currently-running Ruby,
and rubygems-update 3.6 bumped the required Ruby version to `>= 3.1.0`.
This has made it a much bigger hassle to keep supporting Ruby 3.0 in our CI.

Earthfile

@@ -53,8 +53,6 @@ test-bundle:
     RUN ./tests/completion-bundle/test.sh
 
 test-bundle-all:
-    BUILD +test-bundle --RUBY_VERSION=3.0 --RAILS_VERSION="~>6.0"
-    BUILD +test-bundle --RUBY_VERSION=3.0 --RAILS_VERSION="~>7.1.0"
     BUILD +test-bundle --RUBY_VERSION=3.1 --RAILS_VERSION="~>7.0"
     BUILD +test-bundle --RUBY_VERSION=3.2 --RAILS_VERSION="~>7.0"
     BUILD +test-bundle --RUBY_VERSION=3.3 --RAILS_VERSION="~>7.0"
@@ -109,8 +107,6 @@ test-rails:
     RUN ./tests/completion-rails/test.sh
 
 test-rails-all:
-    BUILD +test-rails --RUBY_VERSION=3.0 --RAILS_VERSION="~>6.0"
-    BUILD +test-rails --RUBY_VERSION=3.0 --RAILS_VERSION="~>7.1.0"
     BUILD +test-rails --RUBY_VERSION=3.1 --RAILS_VERSION="~>7.0"
     BUILD +test-rails --RUBY_VERSION=3.2 --RAILS_VERSION="~>7.0"
     BUILD +test-rails --RUBY_VERSION=3.3 --RAILS_VERSION="~>7.0"
@@ -124,8 +120,6 @@ test-rake:
     RUN ./tests/completion-rake/test.sh
 
 test-rake-all:
-    BUILD +test-rake --RUBY_VERSION=3.0 --RAILS_VERSION="~>6.0"
-    BUILD +test-rake --RUBY_VERSION=3.0 --RAILS_VERSION="~>7.1.0"
     BUILD +test-rake --RUBY_VERSION=3.1 --RAILS_VERSION="~>7.0"
     BUILD +test-rake --RUBY_VERSION=3.2 --RAILS_VERSION="~>7.0"
     BUILD +test-rake --RUBY_VERSION=3.3 --RAILS_VERSION="~>7.0"