Mark v1.19.0. (#2099)

* Combine the 1.19.0-rc changelog sections.

* Add some more deprecated and client-breaking entries for stuff that should probably be mentioned.

* Add the start of the blurb.

* Put backticks aroudn the new version number in the blurb.

* Update the blurb. Slight rephrasing of to make it sound less like we were responsible for the SDK, and add a link to it. Also point to a couple of the SDK's releases for further information about the upgrade.

* Move the new changelog entries into the release.

* Add a sentence to the blurb about the bulk asset valuation updates.