Update README.txt to include MariaDB (.txt, not .md)

Considering the technical requirements (https://manual.joomla.org/docs/next/get-started/technical-requirements/) include MariaDB, I humbly suggest the README.txt should include MariaDB too. I removed "SQL Server" from list of databases, as mentioned in another discussion (joomla#43343). There is a separate PR for the .md version of the README.

README.txt

@@ -12,7 +12,7 @@ The Joomla! 5.1 branch is under heavy development and not all links in this docu
 2- What is Joomla?
 	* Joomla! is a Content Management System (CMS) which enables you to build websites and powerful online applications.
 	* It's a free and Open Source software, distributed under the GNU General Public License version 2 or later.
-	* This is a simple and powerful web server application and it requires a server with PHP and either MySQL, PostgreSQL or SQL Server to run.
+	* This is a simple and powerful web server application and it requires a server with PHP and either MySQL, MariaDB or PostgreSQL to run.
 	You can find full technical requirements here: https://downloads.joomla.org/technical-requirements.
 
 3- Is Joomla! for you?