Update beats-ci workers for the test-infra validation

.ci/validateWorkersBeatsCi.groovy

@@ -62,30 +62,30 @@ pipeline {
           axis {
             name 'PLATFORM'
             values (
+              'arm',  // ephemeral workers
+              'immutable && windows-2019',
+              'immutable && windows-2016',
+              'immutable && windows-2012-r2',
+              'immutable && windows-10',
+              'immutable && windows-8',
+              'immutable && windows-2008-r2',
+              'immutable && windows-7-32-bit',
+              'immutable && windows-7',
               'ubuntu && immutable',
-              'ubuntu-1404-i386',
-              'ubuntu-16',
               'ubuntu-18',
               'ubuntu-20',
+              'worker-c07c6107jyw0',  // macOS workers https://beats-ci.elastic.co/label/macosx/
               'worker-c07jc1nzdwym',
               'worker-c07l34n6dwym',
               'worker-c07y20b6jyvy',
-              'worker-c07ll940dwyl',
               'worker-c07y20b9jyvy',
               'worker-c07y20b4jyvy',
               'worker-c07y20bcjyvy',
               'worker-c07mq25jdy3h',
               'worker-c07mq1u7dy3h',
-              'worker-395930',
-              'worker-0a434dec4bdcd060f',
-              'immutable && windows-2019',
-              'immutable && windows-2016',
-              'immutable && windows-2012-r2',
-              'immutable && windows-10',
-              'immutable && windows-8',
-              'immutable && windows-2008-r2',
-              'immutable && windows-7-32-bit',
-              'immutable && windows-7'
+              'worker-h2wdt2qxq6ny',
+              'worker-395930',  // metal workers https://beats-ci.elastic.co/label/metal/
+              'worker-1244230'
             )
           }
         }