Rename the "Example: DBTNG" module

Problem/Motivation

DBTNG was the name given to the database abstraction layer introduced in Drupal 7. Nowadays, that is a no longer used acronym.

Proposed resolution

Change the module name to Example: Database and its machine name to db_example.

I am postponing this, since it should be done in the new branch compatible with Drupal 11.4.