Perform migration

Please tell me, you need to run this script, but not enough experience. How to do it correctly? The initial topic here is https://github.com/RocketChat/Rocket.Chat/issues/14504#issuecomment-493446375, but no one answers.
I tried to write this script in a single v12222.js file and run it, got an error

ReferenceError: Migrations is not defined
at Object. (/root/v12222.js:1:63)
at Module._compile (module.js:652:30)
at Object.Module._extensions..js (module.js:663:10)
at Module.load (module.js:565:32)
at tryModuleLoad (module.js:505:12)
at Function.Module._load (module.js:497:3)
at Function.Module.runMain (module.js:693:10)
at startup (bootstrap_node.js:191:16)
at bootstrap_node.js:612:3
`

Script v122.js that I need to run.
RocketChat.Migrations.add({
version: 122,
up() {
RocketChat.models.Subscriptions.tryDropIndex('u._id_1_name_1_t_1_code_1');
console.log('Fixing ChatSubscription u._id_1_name_1_t_1_code_1');
}
});`

Tell me what I'm doing wrong and how it should be. Thank.

Start by reviewing the referenced v122.js migration and the reported v12222.js execution error, then trace how RocketChat.Migrations is made available when migrations run. Document the correct way to execute this migration and verify that the instructions address the missing Migrations reference.