Bootstrap/Load: Remove the ability to alter the list of environment types in...

Bootstrap/Load: Remove the ability to alter the list of environment types in `wp_get_environment_type()`.

The intention of `wp_get_environment_type()` is to provide a consistent means of identifying the environment type, not of identifying a specific environment.

Actual environments should fit within one of the existing types: `local`, `development`, `staging`, or `production`. That should cover the types that plugins and themes might be concerned about when toggling functionality. 

Props johnbillion, joostdevalk, TimothyBlynJacobs, jeremyfelt, batmoo, claytoncollie, Clorith, markjaquith, garrett-eclipse, GaryJ, elrae.
Merges [48894] and [48895] to the 5.5 branch.
Fixes #50992.
Built from https://develop.svn.wordpress.org/branches/5.5@48896


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48658 1a063a9b-81f0-0310-95a4-ce76da25c4cd