GH CI: remove webdrivers; upgrade selenium-webdriver to latest

Addresses #531

Gemfile

@@ -116,7 +116,6 @@ gem 'exception_notification', '~> 4.4.0'
 gem 'appsignal'
 
 group :test do
-  gem 'webdrivers'
   gem 'm', '~> 1.5.0'
   gem 'minitest'
   gem 'minitest-ci', '~> 3.4.0'

Gemfile.lock

@@ -455,7 +455,7 @@ GEM
       actionpack (>= 5.2)
       railties (>= 5.2)
     retriable (3.1.2)
-    rexml (3.2.5)
+    rexml (3.2.6)
     rgeo (3.0.0)
     rgeo-geojson (2.1.1)
       rgeo (>= 1.0.0)
@@ -507,7 +507,7 @@ GEM
       sprockets-rails
       tilt
     sd_notify (0.1.1)
-    selenium-webdriver (4.10.0)
+    selenium-webdriver (4.12.0)
       rexml (~> 3.2, >= 3.2.5)
       rubyzip (>= 1.2.2, < 3.0)
       websocket (~> 1.0)
@@ -570,10 +570,6 @@ GEM
       activemodel (>= 6.0.0)
       bindex (>= 0.4.0)
       railties (>= 6.0.0)
-    webdrivers (5.2.0)
-      nokogiri (~> 1.6)
-      rubyzip (>= 1.3.0)
-      selenium-webdriver (~> 4.0)
     webpacker (4.3.0)
       activesupport (>= 4.2)
       rack-proxy (>= 0.6.1)
@@ -659,7 +655,6 @@ DEPENDENCIES
   twitter-typeahead-rails (= 0.11.1.pre.corejavascript)
   uglifier (>= 1.3.0)
   web-console
-  webdrivers
   webpacker (~> 4.x)
   whenever (~> 1.0.0)