Administration: Capitalize `Trash` consistently in various messages and comments.

When used as a noun referring to the "virtual" place, `Trash` should be capitalized.

Props garrett-eclipse, aandrewdixon, Presskopp.
Fixes #45317.
Built from https://develop.svn.wordpress.org/trunk@47233


git-svn-id: https://core.svn.wordpress.org/trunk@47033 1a063a9b-81f0-0310-95a4-ce76da25c4cd