Restrict selenium to compatible versions

These are only used by the test suite. But this probably means we won't
be able to support ruby <3.0 for much longer. (2.6 and 2.7 are EOL,
though, so that seems reasonable.)

Gemfile

@@ -7,6 +7,16 @@ gemspec
 # mime-types 3+, required by mail, requires ruby 2.0+
 gem "mime-types", "< 3" if RUBY_VERSION < "2"
 
+if RUBY_VERSION < "2.7"
+  # Ruby 2.7 requirement introduced
+  # https://github.com/SeleniumHQ/selenium/blob/selenium-4.2.0/rb/CHANGES#L25
+  gem "selenium-webdriver", "< 4.2.0", group: :development
+elsif RUBY_VERSION < "2.8"
+  # Ruby 3.0 requirement introduced
+  # https://github.com/SeleniumHQ/selenium/blob/selenium-4.9.1/rb/CHANGES#L5
+  gem "selenium-webdriver", "< 4.9.1", group: :development
+end
+
 #group :development do
 #  gem "pry"
 #end