Upgrade/Install: Convert `Sample Page`, `Hello World`, and `Privacy Policy` to block content.

Merges [43820,43912] from the 5.0 branch to trunk.

Props desrosj, garrett-eclipse, danielbachhuber, dd32, ocean90.
Fixes #45151.


Built from https://develop.svn.wordpress.org/trunk@44168


git-svn-id: https://core.svn.wordpress.org/trunk@43998 1a063a9b-81f0-0310-95a4-ce76da25c4cd