Rename MySQL namespace to Mysql

[SimonFrings]

This pull request changes the React\MySQL namespace to React\Mysql without any additional code adjustments.

Both options are valid when it comes to PSR-4. The specification describes the use of alphabetic characters like this:

Alphabetic characters in the fully qualified class name MAY be any combination of lower case and upper case.

Changing the namespace to React\Mysql aligns with recent changes made in #186 and #187, where we added a Mysql prefix to name the MysqlClient and MysqlResult. This means my suggested PR improves the consistency when it comes to naming our namespace, classes, and variables.

Considering that v0.7.x already includes BC-Breaks from #186 and #187, it could be a perfect time to introduce this additional BC-Break as well.

I'm interested in your thoughts on this 👍

[clue]

@SimonFrings Thanks for looking into this!

I agree typing the FQCN …\MySQL\Mysql… is somewhat awkward and using the name MySQLClient in #186/#147 didn't really feel like an option either, so it probably makes sense to consistently go with the Mysql casing also for the classname 👍

[WyriHaximus]

I'm interested in your thoughts on this 👍

Meh but consistency is more important IMHO