Add upcoming deprecations

[locks]

Complements emberjs/ember.js#14590.

[jenweber]

Please remove backticks around the id, for consistent formatting

[serenaf]

Original PR for reference :) emberjs/ember.js#14590

[serenaf]

@locks According to this https://www.emberjs.com/blog/2018/01/01/ember-2-18-released.html#toc_changes-in-ember-js-2-18 it is since 2.18.0 and until 3.5.0

[jenweber]

@locks can you remove backticks around the id? It is a small thing but makes future CSS easier. Also while you're at it, can you promote Ember 2 legacy up a level, so three ### instead of four? It ought to be its own section now that I look at it.

I totally used targetObject in my apps. Oops.

[locks]

@jenweber done

[jenweber]

I noticed an issue with using backticks in the deprecation title too, but it's a styling problem that affects other listings so I'll open a separate PR/Issue