ci: update chrome and firefox saucelabs versions (angular#29518)

Updates the browser versions of Chrome and Firefox to the latest releases. This
matches what we claim to support according to https://angular.io/guide/browser-support.

PR Close angular#29518

browser-providers.conf.js

@@ -43,10 +43,10 @@ var customLaunchers = {
   'DartiumWithWebPlatform':
       {base: 'Dartium', flags: ['--enable-experimental-web-platform-features']},
   'ChromeNoSandbox': {base: 'Chrome', flags: ['--no-sandbox']},
-  'SL_CHROME': {base: 'SauceLabs', browserName: 'chrome', version: '67'},
+  'SL_CHROME': {base: 'SauceLabs', browserName: 'chrome', version: 'latest'},
   'SL_CHROMEBETA': {base: 'SauceLabs', browserName: 'chrome', version: 'beta'},
   'SL_CHROMEDEV': {base: 'SauceLabs', browserName: 'chrome', version: 'dev'},
-  'SL_FIREFOX': {base: 'SauceLabs', browserName: 'firefox', version: '60'},
+  'SL_FIREFOX': {base: 'SauceLabs', browserName: 'firefox', version: 'latest'},
   'SL_FIREFOXBETA':
       {base: 'SauceLabs', platform: 'Windows 10', browserName: 'firefox', version: 'beta'},
   'SL_FIREFOXDEV':