[DOC beta] Deprecation link for Ember.Component.reopen

[Glarregle]

Should wait for ember-learn/deprecation-app#948

[jenweber]

The deprecation guide has been merged, so this can move forward.

@Glarregle please mark your commit with [DOC beta] at the beginning of the commit message. This will help us make sure it gets in for the next release.

[Glarregle]

Thanks @jenweber! Ready!

[mixonic]

This needs to be rolled back. This deprecation was targeting 4.0, and we want to remove the functionality in 4.0. Followup from @chancancode is:

• PR to move the deprecation guide to 3.x (also the link-to.disabled-when ones)
• PR to change "until" back to 4.0 on master and beta, as well as updating the URL to 3.x
• PR to backport adding the URL (and since) to 3.28

[jenweber]

@mixonic PR to fix the versions #19823

Sorry for the confusion @Glarregle, we gave you the wrong instructions about version numbers. Thanks again for your help with fixing missing info.