Change name pattern of download files

[olafgrabienski]

The name pattern for download files on the translation server is backdropcms-VERSION.LANGCODE.po. @quicksketch wrote in backdrop/backdrop-issues#3674 (comment):

it raised my eyebrows that the name of the downloaded file was "backdropcms" instead of just "backdrop". Inside our own codebase, we always use the shortened version without "cms" for function names, settings, etc.

When backdrop/backdrop-issues#3674 is fixed, let's drop the cms part of the name pattern and change it to backdrop-VERSION.LANGCODE.po.

[olafgrabienski]

In the meantime, backdrop/backdrop-issues#3674 has been fixed, so we can drop the cms part of the file names. There was however another core issue (backdrop/backdrop-issues#3974) with translation file names recently. I'd prefer to wait with the name pattern change here until the recent core issue is fixed and part of a new Backdrop release. We should test then, if Backdrop can still be installed with a language other than English (as expected).