ci: lock chrome and firefox versions for saucelabs (angular#29529)

With 0f1da49, the Chrome beta job has
been disabled because a new Chrome beta has been released, but Saucelabs
didn't support a chromedriver that is compatible with that given beta version.

Now the topic of pinning these external browsers to a specific version came up. In order to make the build less prone to unexpected new versions, we need to
_permanently_ disable the Chrome beta browser. Otherwise pinning `SL_CHROME`
to a specific version does not statisfy the requirement of making the CI jobs
more deterministic.

See original discussion: angular#29518 (comment)

Chrome can be pinned to Chrome v73 (latest stable version at time of this commit)
Firefox can be pinned to Firefox v65 (latest stable version available in Saucelabs platform)

PR Close angular#29529

Author: devversion

browser-providers.conf.js

@@ -14,9 +14,9 @@
 var CIconfiguration = {
   'Chrome': {unitTest: {target: 'SL', required: true}, e2e: {target: null, required: true}},
   'Firefox': {unitTest: {target: 'SL', required: true}, e2e: {target: null, required: true}},
-  // Temporarily disabled because Saucelabs now runs Chrome v74 as beta version. Unfortunately
-  // Saucelabs doesn't provide a Chromedriver version that works for that beta version though.
-  // TODO(FW-1205): Re-enable once Saucelabs supports Chromedriver v74.0.3729.6.
+  // Disabled because using the "beta" channel of Chrome can cause non-deterministic CI results.
+  // e.g. a new chrome beta version has been released, but the Saucelabs selenium server does
+  // not provide a chromedriver version that is compatible with the new beta.
   'ChromeBeta': {unitTest: {target: 'SL', required: false}, e2e: {target: null, required: false}},
   'ChromeDev': {unitTest: {target: 'SL', required: false}, e2e: {target: null, required: true}},
   // FirefoxBeta and FirefoxDev should be target:'BS' or target:'SL', and required:true
@@ -46,10 +46,10 @@ var customLaunchers = {
   'DartiumWithWebPlatform':
       {base: 'Dartium', flags: ['--enable-experimental-web-platform-features']},
   'ChromeNoSandbox': {base: 'Chrome', flags: ['--no-sandbox']},
-  'SL_CHROME': {base: 'SauceLabs', browserName: 'chrome', version: 'latest'},
+  'SL_CHROME': {base: 'SauceLabs', browserName: 'chrome', version: '73'},
   'SL_CHROMEBETA': {base: 'SauceLabs', browserName: 'chrome', version: 'beta'},
   'SL_CHROMEDEV': {base: 'SauceLabs', browserName: 'chrome', version: 'dev'},
-  'SL_FIREFOX': {base: 'SauceLabs', browserName: 'firefox', version: 'latest'},
+  'SL_FIREFOX': {base: 'SauceLabs', browserName: 'firefox', version: '65'},
   'SL_FIREFOXBETA':
       {base: 'SauceLabs', platform: 'Windows 10', browserName: 'firefox', version: 'beta'},
   'SL_FIREFOXDEV':