Shorten method name to `Gitlab::Database::EachDatabase.each_connection`

What does this MR do and why?

Related to #413918 (closed)

This MR renames the method

Gitlab::Database::EachDatabase.each_database_connection

and it's alias

Gitlab::Database::EachDatabase.each_db_connection

to a shorter:

Gitlab::Database::EachDatabase.each_connection

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Related to #413918 (closed)