Add a guide to updating from old versions of Yo

[JoshuaKGoldberg]

Coming over from yeoman/generator#1294, I'm starting to collect common questions around updating old generators:

• What to replace this.npmInstall with?
  • In general: what to do with deprecated commands?
• If a composed sub-generator used this.options, how to make the parent generator use that now?
• (likely more to be filled out soon)

Proposal: maybe it'd be good to have a general guide like "Migrating from pre-2024 Yeoman libraries"?