refactor(core): mark `@defer` APIs as stable

[AndrewKushnir]

This commit removes the @developerPreview annotation from the @defer APIs, effectively promoting them (and the entire feature!) to stable. Relevant guides on AIO and ADEV do not have a "developer preview" note, so no changes to the guides were required.

Thanks everyone for providing the feedback and helping to improve this feature!

Special thanks (and congrats!) to @crisbeto @clydin @thePunderWoman for working on this feature and getting it to the stable state 🎉

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

[AndrewKushnir]

This PR was merged into the repository by commit 7645325d4605fd42fd48dab4e50f75f0c61a4ee9.