[UX] Rename name "Filtered HTML" input format to "Basic"

Let's remove the word "HTML" from the Filtered HTML format name. Since you can't enter HTML here anymore, having a format name that indicates that you can is confusing.

I'm proposing we change the name of this format to "Basic".

Related issue: [UX] Change the name of the "Full HTML" text format to "Raw HTML" - https://github.com/backdrop/backdrop-issues/issues/4499 - EDIT [This related issue in done and merged into core]

PR: https://github.com/backdrop/backdrop/pull/2885